UPTEC Q10004
Examensarbete 30 hp Maj 2010
Influence of composition, grain size and manufacture process on the anisotropy of tube materials
Daniel Gullberg
Teknisk- naturvetenskaplig fakultet UTH-enheten
Besöksadress:
Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0
Postadress:
Box 536 751 21 Uppsala
Telefon:
018 – 471 30 03
Telefax:
018 – 471 30 00
Hemsida:
http://www.teknat.uu.se/student
Abstract
Influence of composition, grain size and manufacture process on the anisotropy of tube materials
Daniel Gullberg
A problem with cold pilgered tubes for OCTG applications is that they can get anisotropic properties with regard to yield strength. One source of anisotropy is texture that is developed during the cold deformation. EBSD measurements have been made on several austenitic stainless steels with different deformations to see what influence the composition has on the texture formation. The same
measurements were used to study the influence of grain size on texture formation.
The conclusion was that the composition can have an impact on the texture and hence has potential to also affect the anisotropy. The differences in texture cannot be associated with a specific alloying element, but is rather a synergetic effect. It was also concluded that grain structure has no strong influence on texture formation. An evaluation of three different tool designs used for cold pilgering was made. The designs evaluated are referred to as design A, B and C. EBSD measurements showed large deviations in texture in the middle of the wall compared to close to the surface of pilgered OCTG. However, the measurements showed no large differences between the three designs and the texture could not be coupled to the anisotropy.
Tryckt av: Ångströmlaboratoriet, Uppsala Universitet Sponsor: Sandvik Materials Technology
ISSN: 1401-5773, UPTEC Q10004 Examinator: Åsa Kassman Rudolphi Ämnesgranskare: Urban Wiklund
Handledare: Guocai Chai, Katarina Persson, Anders Nyström och Fredrik Meurling
Inverkan av sammansättning, kornstorlek och tillverkningsprocess på anisotropi i rörmaterial
Daniel Gullberg
För att ta upp olja från oljekällor i havsbottnen används stålrör. I takt med att oljetillgångarna i jordskorpan minskar måste tekniken utvecklas för att kunna komma åt oljan även i tuffa miljöer. Miljön där oljan finns är ofta kraftigt korrosiv och de mekaniska påfrestningarna på rören är höga. Det krävs speciella stål som har hög hållfasthet och mycket bra
korrosionsegenskaper. För att få tillräcklig hållfasthet krävs det oftast en kallbearbetning av rören. Det finns flera olika sätt att åstadkomma kallbearbetningen. De vanligaste sätten är kalldragning eller stegvalsning. Sandvik AB är ett av de företag som tillverkar rostfria rör för den här tillämpningen. Sandvik AB stegvalsar sina rör. Ett problem som uppstår vid
stegvalsning men även vid kalldragning är anisotropi. Anisotropi innebär i det här fallet att hållfastheten (sträckgränsen) blir olika i olika riktningar. Oftast blir sträckgränsen parallellt med röret högre än vad den blir transversellt i röret, men även andra fall förekommer. Det finns mycket att vinna på att bli av med anisotropin. För det första så är det lättare att göra hållfasthetsberäkningar på rören om sträckgränsen är lika i alla riktningar. För det andra så skulle materialåtgången kunna minskas vilket dels har ekonomiska fördelar och dels praktiska fördelar eftersom rören skulle bli lättare.
Syfte
Syftet med den här studien är att undersöka vilken inverkan sammansättningen, kornstorleken och tillverkningsprocessen har på anisotropin. En av källorna till anisotropi är textur i
materialet. För att förstå vad textur är och hur texturen kan påverka anisotropin är det viktigt att först känna till hur ett metalliskt material är uppbyggt. I den här undersökningen
förekommer endast austenitiska rostfria stål. Dessa stål består av endast en fas där legeringselementen är inlösta. Stålet är uppbyggt av korn i storleksordningen tiotalet
mikrometer. Varje korn är en kristall, det vill säga atomerna i kristallen är uppradade efter ett visst mönster som är typiskt för den fasen. Textur är hur de olika kristallerna eller kornen är orienterade i förhållande till varandra. Varje kristall i sig är anisotropisk och har olika egenskaper och styrka i olika riktningar. Så om fördelningen av kristallorienteringar inte är helt slumpvis kommer även materialet bli anisotropt.
Utförda undersökningar och resultat
För att se hur sammansättnigen påverkar texturen och i sin tur anisotropin har några olika austenitiska stål valts ut där sammansättnigen varierar på ett sådant sätt att det skulle vara möjligt att dra slutsatser om inverkan av specifika legeringselement. De element som ansågs mest intressanta var kväve, molybden och koppar. I tillverkningen av rör uppstår texturen vid stegvalsningen. Stegvalsningsprocessen är komplicerad och för att förenkla försöket
deformerades proverna istället genom dragning i en dragprovningsmaskin. Varje stålsort deformerades till fem olika deformationsgrader varvid texturmätningar genomfördes.
Texturmätningarna gjordes med EBSD (electron backscatter diffraction), som är en
analysmetod där diffraktionsmönstret från en elektronstråle detekteras och ger information om
kristallorienteringen i den punkt elektronstrålen träffar provet. Resultaten visade att vid
dragningen av proverna vrids kornen så att vissa specifika kristallriktningar linjerar sig parallellt med dragriktningen. Dessa riktningar benämns <111> och <100>. Två av stålen visade avvikande texturbildning jämfört med de andra med en mer långsam vridning vilket resulterade i en svagare textur. Det gick inte att koppla den här effekten till något specifikt legeringselement utan är snarare en synergieffekt. Eftersom det går att se skillnader i texturen är det troligt att det även skulle resultera i skillnader i anisotropi.
Ett liknande försök genomfördes för att undersöka vilken inverka kornstorleken kan ha på anisotropin hos stegvalsade produkter. Genom att värmebehandla material kunde prover med samma sammansättning fast med olika kornstorlek undersökas. Resultatet blev att det inte gick att se några skillnader i texturutvecklingen utifrån kornstorleken.
Slutligen gjordes en utvärdering av tre olika valsverktygsdesigner för att se vilken påverkan valsverktyget har på anisotropin. De tre designerna benämns A, B och C. Design B deformerar röret mer homogent än de övriga mot slutet av valsningen. Design C reducerar väggen mer i förhållande till diametern än de övriga designerna i början av valsningen och tvärt om mot slutet. För alla designerna har röret samma ingående och utgående dimension.
Texturmätningar har gjorts på rören som visar att texturen är annorlunda i mitten av rörväggen jämfört med nära ytter och inner ytan. Texturen är däremot av samma typ för alla tre
designerna. Även om det går att se små skillnader på bland annat styrkan hos texturen så går inte dessa skillnader att koppla till uppmätt anisotropi för dessa rör. Inte heller anisotropin visar några stora skillnader för de tre designerna.
Slutsatser
Inverkan av sammansättning, kornstorlek och tillverkningsprocess på anisotropin har
undersökts. Förutom en viss inverkan på texturen för olika sammansättningar så kunde ingen inverkan påvisas. En förändrad textur ger troligen också skillnad i anisotropi.
Examensarbete 30 hp på civilingenjörsprogrammet Teknisk fysik med materialvetenskap
Uppsala universitet, maj 2010
Acknowledgements
First of all, I would like to thank my mentors at Sandvik Guocai Chai, Katarina Persson,
Anders Nyström, and Fredrik Meurling for their help and guidance during this thesis. Most of all I want to thank Guocai for giving me this opportunity and for supporting me the whole way with ideas and comments.
Secondly, I would like to thank Sorina Ciurea for her help regarding the EBSD measurements and for valuable discussions of the results.
During my thesis I have received a lot of help from numerous people at Sandvik. I would like to thank all that have helped me or contributed with specimen fabrication, measurements, sample
preparation, etching, and knowledge about the equipment. I specially appreciate that I could use the EBSD equipment during the weekends.
I also want to thank Göran Engberg and Stefan Jonsson for their time. With their expertise they contributed with valuable discussions about the texture formation.
Last of all, I want to thank my family and friends, especially Maryam, for their support.
Table of Content
1. Introduction ... 1
1.1 Objective... 1
2. Literature Study ... 3
2.1 Well Construction and OCTG Materials ... 3
2.1.1 Requirements for OCTG Materials ... 4
2.1.2 Austenitic and Duplex Stainless Steels ... 4
2.1.3 Influence of Alloying Elements on the Properties ... 5
2.2 Anisotropy ... 6
2.2.1 Deformation and Dislocations ... 6
2.2.2 Texture and Anisotropy ... 9
2.2.3 Presentation of Textures ... 9
2.2.4 Analysis Method for Texture and Anisotropy ... 10
2.2.5 Parameters that Affect Texture and Anisotropy ... 12
2.3 Manufacture of OCTG ... 14
2.3.1 Methods for Manufacture of OCTG ... 14
2.3.2 Plastic Deformation Mechanisms and Anisotropy during Cold Working ... 16
2.4 Previous Work ... 17
3. Test Material ... 18
3.1 Steel Grades... 18
3.1.1 Sanicro 28 ... 18
3.1.2 Sanicro 29 ... 19
3.1.3 Sanicro 30 ... 19
3.1.4 Sanicro 41 ... 19
3.1.5 3R60 ... 19
3.1.6 3R69 ... 20
3.1.7 254 SMO ... 20
3.2 Heat Treatment and Grain Size ... 20
3.3 Test Samples ... 21
4 Experimental ... 22
4.1 Sample Straining ... 22
4.2 EBSD... 23
4.3 LOM ... 24
4.4 Vickers Hardness ... 24
4.5 Rockwell C Hardness ... 24
5 Results and Discussion... 25
5.1 Microstructure of Reference Samples (LOM)... 25
5.2 Microstructure After Deformation for Sanicro 29 (LOM) ... 26
5.3 Microstructure of Reference Samples (EBSD) ... 27
5.4 Texture Formation (EBSD) ... 29
5.4.1 Inverse Pole Figures ... 29
5.4.2 Specific Orientation Measurements ... 33
5.5 Substructure of Deformed Samples (EBSD) ... 39
5.6 Strain Hardening Coefficient ... 41
5.7 Influence of Composition and Grain Structure on Texture ... 41
5.8 TEM ... 42
5.9 Misorientation ... 43
5.10 Tool Design Evaluation ... 44
5.10.1 Hardness Variations (Rockwell C) ... 44
5.10.2 Texture (EBSD) ... 46
5.10.3 Anisotropy Values ... 48
6. Conclusions ... 49
7. References ... 50
List of Abbreviations and Symbols
OCTG Oil country tubular gods EBSD Electron backscatter diffraction SEM Scanning electron microscopy FCC Face centered cubic
HCP Hexagonal close packed SFE Stacking fault energy
sfe Stacking fault energy per area unit
b Burgers vector
b1, b2 Burgers vectors of Shockley partials
Fr Repulsive force between two partial dislocations
G Shear modulus
pi = 3.14…
Fa Attractive force between two partial dislocations
Shear force component
ODF Orientation distribution function x, y, z Coordination system of the crystal
RD, TD, ND Coordination system of the sample (rolling, transversal and normal direction)
2 1, ,
Euler angles
B Bragg angle
dhkl Distance between parallel planes
R Resolved shear stress
Angle between slip plane normal and stress direction
Angle between slip and stress direction
Stress
CNC Computerized numerical control LOM Light optical microscopy
P Load in kg
d Average diameter of a increment
HV5 Hardness value from a Vickers test with load 5 kg IPF Inverse pole figure
TEM Transmission electron microscopy
Rp0.2 Tension in the material at strain offset 0.002 (yield strength)
1
1. Introduction
The requirements for oil country tubular gods (OCTG) materials are getting higher at the same rate as the easily exploited wells are running out. The oil is often found deep in the seabed at depths of up to 7 km with pressures up to 1050 bar, possibly even higher [1]. In order to reach the required strength the material needs to be cold-worked. When performing the cold-working, the tube can get anisotropic properties. Normally the tensile strength is higher in the lengthwise direction of the tube than in the transversal direction. This effect is a problem due to the different types of pressure subjected to the tube when used in a well, see figure 1.
Figure 1. Illustration of the forces subjected to the tube; a)tension b)compression c)collapse and d)burst The tubes have high strength requirements for all the load cases; tension, compression, collapse, and burst. It is not possible to compensate for the low transversal yield strength by increasing the total strength of the tube because this will cause corrosion problems, exceeding the limits of hardness and give a brittle material [2]. The lower transversal yield strength can be compensated by increasing the wall thickness, resulting in a more expensive and heavier tube. Large savings can be made by
decreasing material usage and make the dimensioning easier if the anisotropy could be reduced.
One of the sources of anisotropy in the case of cold-working is texture formation. Texture is when the orientation of the crystals is not random. Almost all metallic materials, even if the material has not been cold worked, have texture to some extent. This type of texture forms when the alloy is made, due to its shape, or the cooling direction in the material, but it is often weak and the material can thus be regarded as uniform. If the material is cold-worked deformation texture is formed as a consequence of deformation mechanisms, which are dependent on the composition and
temperature. The deformation causes movements in the crystals and rotations of the crystal directions.
1.1 Objective
The aim of this thesis is to give an understanding of how parameters like composition, grain size, and manufacture process affect the anisotropy formation in austenitic stainless steels. The results can perhaps help to get ideas of how to prevent texture formation in austenitic materials for OCTG applications. In a long term perspective the aim is to decrease the anisotropy levels in pilgered tubing.
The main investigation in this thesis will be to measure texture formed during tensile deformation at room temperature for several austenitic stainless steels in order to see what influence the chemical
2
composition has on texture formation, if any at all. The texture measurements will be carried out using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). The same type of measurement will be used to investigate if the grain size can have an influence on the texture.
Three pilgered tubes from the same heat but pilgered with different tool designs will be compared to see if one of them is better than the other with respect to texture or homogeneity.
3
2. Literature Study
The purpose of this section is to give a deeper understanding of anisotropy in tube materials and to give adequate information to be able to follow the results and discussion. At the end of this section there is a short review of published scientific articles concerning texture formation and deformation mechanisms.
2.1 Well Construction and OCTG Materials
The main application of OCTG is to transport oil and gas from the well to the sea bed. At the top of each hole down to the well there is a wellhead. When the oil or gas reaches the wellhead, it enters an oil transportation system made up like a network that transports the oil to main land or to a platform at the surface, see figure 2. In the hole there are two layers of OCTG, the outermost tube protects the inner tube that transports the oil to the sea bed. The tubing is put together using a threaded coupling. Other names for OCTG are down-hole tubing or production tubing [1].
Figure 2. Well construction: illustration of a) how the oil is transported to the surface and b) the OCTG usage.
Normally ordinary carbon steels are used as OCTG. However, there are many oil wells where ordinary carbon steels are not sufficient due to highly corrosive environments or other requirements. Some of the aspects that need to be considered when choosing OCTG material are temperature, partial pressure of H2S and CO2, total pressure, pH-value, and chloride content. Carbon steel or low alloyed steel is the cheapest alternative. Austenitic or duplex stainless steels can be cold worked to reach very high strength at the same time as they have good corrosion resistance. Austenitic stainless steels are expensive due to their high nickel content but are not as sensitive to H2S as duplex stainless steels. Austenitic stainless steels are chosen if the partial pressure of H2S exceeds 3 psi [1].
4
2.1.1 Requirements for OCTG Materials
OCTG are affected by different forces when used, such as tension, compression, internal pressure and collapse. It is important to have the proper strength since a lower strength does not meet the strength requirements and a higher strength gives a more brittle material and can cause corrosion problems.
To reach the required strength OCTG tubes normally need to be cold-worked. This can be done either by cold pilgering or cold-drawing. This treatment can however give an undesired anisotropy that can affect the design of the OCTG tube. When designing OCTG tubes the internal pressure is normally calculated using the (American Petroleum Institute) API 5C3 formula that is based on the tensile yield strength in the transversal direction and the tube dimensions. The transversal yield strength is in the case of anisotropy normally lower than the longitudinal yield strength, which makes it necessary to increase the wall thickness of the tube compared to if the material was isotropic. The tube will thus require more material especially expensive nickel, furthermore it will make the tubes heavier [2].
2.1.2 Austenitic and Duplex Stainless Steels
Stainless steels are normally considered to be iron-based alloys with high fractions of chromium (10.5 wt %). When the chromium exceeds 10.5-12 wt % it enables the forming of a thin self healing
chromium oxide film that prevents further oxidation of the steel [3]. Other frequently used alloying elements in stainless steels are nickel, manganese and silicon [4]. The stainless steels can be divided into five basic groups where austenitic and duplex stainless steels are the two commonly used for OCTG applications.
The development of stainless steels started independently both in Germany by the company Krupp and in Sheffield, Great Britain around 1910. Since then the use of nitrogen as an alloying element and new process techniques have improved the steels. Due to the nitrogen’s stabilizing effect on the austenitic phase, higher amounts of chromium and molybdenum can be used to further improve the corrosion resistance [3].
The austenitic steels are as the name implies, consisting of one phase, the austenite phase. This is a face centered cubic (FCC) structure which gives the material high ductility and good formability compared to other groups of stainless steel. There are three ways to harden a stable austenitic material, strain hardening, grain refinement through cold working followed by recrystallization, and solid solution hardening [4].
The duplex steels are consisting of two phases in significant properties. Duplex stainless steels are normally ferritic/austenitic steels with 30 to 70 % ferrite. The first commercial products were marketed around 1930 and were then used for autoclaves, valves and coolers. Since then several improvements have been made on the corrosion resistance. The advantages over austenitic stainless steels are better resistance to chloride stress corrosion, higher strength and the low nickel content [5].
5
2.1.3 Influence of Alloying Elements on the Properties
All steel grades in this thesis are austenitic stainless steels, this means that there are only supposed to be one phase present, namely austenite. When alloying these steels the solute is added without changing the phase of the material or causing other phases to form, this is referred to as solid solution alloying. How much of a solute that can be solved in the solvent depends on several things;
size of the solute atom, crystal structure, electronegativity and valences. When a solute is added to a solvent the atoms are placed either interstitially between the host atoms or substitutionally taking the place of a host atom [6], se figure 3.
Figure 3. Illustration of how alloy atoms are placed in the solvent at solid solution.
The most important reason for further development of stainless steel has been to improve the corrosion resistance. The most important element for this is chromium, but many other elements also affect the corrosion properties. The alloying elements also have influence on the microstructure and are divided into two groups either ferrite or austenite stabilizers. C, N, Mn, Co and Cu are typical austenite stabilizers, while Si, V, Cr, Mo and W are ferrite stabilizers [3].
This thesis will discuss what influence N, Cu and Mo have on the structure of austenitic stainless steels. It is therefore of interest to briefly describe their effect as alloying elements.
Nitrogen is a strong austenite stabilizing element that makes it possible to add more Cr and Mo to the alloy. A negative aspect of this is that it increases the sensitivity of precipitation of undesired intermetallic phases and nitrides. Nitrogen gives an increase in yield strength in austenitic steel due to three reasons. First of all, more active glide planes are necessary for the same plastic strain, secondly, more intersections of crossing active glide planes develop which act as dislocation barrier, and last, dislocation barriers are more effective in nitrogen alloyed steels. Nitrogen also has a positive effect on corrosion resistance, particularly on pitting corrosion. Nitrogen neutralizes the H+ ions forming NH4+
ions raising the pH-value, this enhances repasivation of the pit [3]. It is unclear if nitrogen raises or decreases the stacking fault energy (SFE). Nitrogen has been shown to increase the formation of planar dislocations in austenitic steels generally associated with low stacking fault energy materials [7].
Copper may increase the corrosion resistance to general corrosion in sulphuric acid and phosphoric acid environments. Copper may also enable considerable hardening, without excessive loss of
ductility in duplex steels. If copper diffuses to grain boundaries it may cause embrittlement [3]. Steels are rarely alloyed with copper and when they are the amount is only exceeding one percent for special alloys [8].
6
Molybdenum has a ferrite stabilizing and strengthening effect. With high Mo content the structural stability of stainless steel may be lowered. This may cause precipitation during high temperature exposure leading to embrittlement of the material. Molybdenum has the benefit to improve resistance to pitting and crevice corrosion in chloride environments [3]. In OCTG applications Mo is important in respect of stress corrosion cracking in H2S environments [1].
2.2 Anisotropy
A material is anisotropic when its properties depend on how the sample is orientated relative to the direction of the measurement. In this thesis it is important to understand what anisotropy is and what causes anisotropy.
2.2.1 Deformation and Dislocations
An FCC material has two basic methods of deformation; slip and twinning. It is mainly the materials stacking fault energy (sfe) that determine if slip or twinning will be the dominating mechanism. If the SFE is low it will be difficult for cross-slip to occur, and that will make it hard for the deformation to form by slip alone and will lead to twinning [9]. Stacking fault energy will be described deeper later in this report.
To be able to understand how the stacking fault energy influences the deformation it is necessary to first know more about dislocations. In metals as in all materials the atoms are arranged in a way so that they will reach as low energy state as possible, this gives the metal its structure. Which structure that is most beneficial is different for different metals and alloys. This thesis centers around
austenitic stainless steels which have an FCC structure, see figure 4.
Figure 4. The face centered cubic structure where the densely packed planes are shown.
Atoms in a metal are strongly bound to each other and a perfect lattice has a very high strength, much higher than what a normal metal is showing. The reason for this is dislocations, which are linear crystal defects. When the metal is deformed it is due to movement of the dislocations causing the material to change shape. It will be much easier to push one atom at the time to a different place than to move all at once [10], see figure 5.
7
Figure 5. Dislocation movement, the extra half plane is moving to the right on the slip plane illustrating how dislocations are moving in the metal, one step at the time.
Dislocations are normally propagating in the direction in the crystal that is the most favorable from an energy perspective. The planes where the dislocations are moving are called slip planes. The slip plane is often the densest plane. In an FCC crystal, the most dense plane is a {111} plane and the most dense direction in that plane is a <110> direction. The slip system in an FCC crystal is therefore
<110>{111} [11].
In an FCC crystal the atomic layers are stacked in a sequence; ABCABCABC…, a deviation from this stacking sequence is a stacking fault. The normal burger’s vector b in an FCC material is <110> type and reaches between two identical positions regarding to the stacking sequence, see figure 6.
For some dislocations it is more energy efficient to divide into Shockley partials. This means that the movement of the atom will take another way, first one slide to a position different from the first regarding to stacking sequence, and then another slide to the final position. The dissociation will only occur if Frank’s rule:
2 2 2 1
2 b b
b Equation 1
is satisfied, meaning that the energy is lowered [10].
Figure 6. Stacking positions in FCC lattice and dissociation into Shockley partials of a burgers vector.
8
The intermediate position will result in a stacking fault, making a change in the stacking sequence.
The width of the stacking fault will be balanced through one repulsive force between the two partial dislocations, and one attracting force due to the energy increase of the stacking fault area. The repulsive force Fr for each length unit can be written as:
d b Fr Gb
2
2
1 Equation 2where G is the shear modulus and d is the distance between the two dislocations b1 and b2 according to figure 7.
Figure 7. Illustration of how the distance of the partial dislocations is balanced by repulsive and attracting forces.
The attracting force is equal to the stacking fault energy per unit area sfe and the distance between the two dislocations is given by:
sfe
b b d G
2
2 1
Equation 3When one of the partial dislocations gets locked up there will be a shear force component in the direction of the burger’s vector reducing the distance between the Shockley partials, the attractive force can be written:
b2
Fa
Equation 4In order for the dislocation to move past the obstacle the dislocation can climb around it, but this is only possible for a pure dislocation (not for partial dislocations). The force acquired to close the dissociated partials and eliminate the stacking fault is increasing with decreasing stacking fault energy according to equations 2 and 4 [11]. This is the reason that FCC materials with low stacking fault energy have a tendency to form twins instead of deforming by slip.
9
2.2.2 Texture and Anisotropy
A single grain or a single crystal of a metallic material is anisotropic, that is its properties depend on the direction. The typical property is the strength of the material. When the material is deformed slips will occur in the crystal due to dislocation movements. This will happen in the crystals preferred slip plane. Depending on how the slip planes are orientated relative to the deformation direction the material will show different strength.
Texture is when the distribution of crystal orientations in a poly crystal is not random. This causes anisotropy for the same reason as for the single crystal. There are many types of textures or distributions of orientations and several reasons for them. The two main reasons for texture formation are annealing and deformation, this thesis will focus on deformation texture. To make it simple deformation texture can be divided in rolling texture and fiber texture. As the names imply one is normally formed when rolling and one when deforming in one direction. How the texture is formed will be explained later in this thesis report.
2.2.3 Presentation of Textures
When investigating structure and texture it can be very useful to be able to represent planes, directions and angles in a two-dimensional diagram. The most frequently used way of doing this is the stereographic projection. The crystal that should be represented is placed in the centre of a reference sphere. The reference sphere is cut by an equatorial plane on which the two-dimensional projection is made. To get the projection of a direction (e.g. a plane normal) a straight line is drawn from the center of the crystal in the specific direction to a point p on the reference sphere. Then a line is drawn from the south pole of the reference sphere to the point p and its intersection with the equatorial plane is the stereographic projection, see figure 8. The projection can also be made for planes which result in a great circle [12].
Figure 8. Illustration of the stereographic projection.
A pole figure is constructed by making the projection of a certain crystallographic direction (normally 111, 101 or 001 for FCC materials) for all measurement points and then calculating the concentration of poles in every point on the projection.
An inverse pole figure is made by adding the crystallographic orientation parallel to the normal of the sample surface in a 001 standard projection for every measurement point. The concentration of orientations is then calculated in the same way as for the pole figure. A 001 standard projection is a stereographic projection of the most common directions for a perfect single crystal with its 001
10
direction parallel to the plane normal. Due to crystal symmetry all orientations in an FCC material can be represented within a small triangular area of the total projection, see figure 9.
Figure 9. 001 standard projection of an FCC material used for constructing an inverse pole figure. All crystallographic orientations can be presented in the gray area or in any of the other triangles if symmetry is used.
A more complete way of presenting the texture is by an orientation distribution function (ODF). The coordinate system of the crystal is related to the coordinate system of the sample by three Euler angles. To align the crystal coordinate system x, y and z with the sample coordinate system RD, TD and ND it takes three rotations one for each Euler angle. First the coordinate system of the crystal is rotated an angle 1 around ND so that z is parallel with the ND TD plane. Then a rotation around TD is made with an angle so that z is parallel to ND. Last a rotation, 2, is made around ND to align x with RD [11]. In a cubic system all Euler angles are between zero and ninety degrees. The three Euler angles put up a three-dimensional space named the Euler space. Texture is normally presented as two-dimensional slices of the Euler space were one of the angles is held constant.
2.2.4 Analysis Method for Texture and Anisotropy
EBSD (electron back-scatter diffraction) is an add-on package to an SEM (scanning electron
microscopy) which has an ability to obtain microstructure-level information. Most other methods like X-ray or neutron diffraction give diffraction intensities that are characteristic for a large contiguous sample volume. The EBSD measurement gets the crystallographic information from a small sampled volume, which usually is an individual crystallite. The orientation in such a small volume can be taken to be uniform. The pattern gained from the EBSD analysis is named Kikuchi pattern. The
determination of the crystallographic orientation starts with indexing the Kikuchi pattern and then the relative position of the poles with respect to an external reference frame is determined. This can be made automatically by a computer, measuring and indexing several points per second [13].
The Kikuchi pattern is a gnomic projection. All poles are projected on a plane that is a tangent to the north pole of the reference sphere, see figure 10. The computer identifies the desired direction and adds intensity in the pole figure according to the stereographic projection.
11
Figure 10. Illustration of a gnomic projection which is a direct projection on to the projection plane.
The Kikuchi pattern forms due to electron diffraction when electrons interact with the atom planes according to Bragg’s law. When electrons from an electron beam in a SEM enter a sample they are diffusely scattered in all directions. This means that each plane in the crystal is hit by electrons in the Bragg angle B. They can then undergo elastic scattering and form diffusion patterns when they interact with elastically scattered electrons from planes in the same family. The diffraction pattern is forming a cone known as the Kossel cone about the normal of the reflecting plane with the apex angle 180 2B. The reason for this is that the electrons are coming from all directions. One other effect from this is that two cones are forming from each plane, one on each side, see figure 11.
Figure 11. The left picture shows the Kossel cone and the right shows how two cones form from each family of planes.
The Bragg angle is about 0.5°, this means that the Kossel cones are rather flat and when they are projected on a screen they will appear as two parallel lines, Kikuchi-lines. The width between the lines is proportional to the distance between the planes dhkl. Each crystal structure has a specific pattern consisting of pair of lines or bands where each band represents a specific plane and the points where the bands cross represent certain directions in the crystal. By indexing the different planes (the Kikuchi-lines on the screen) it is possible to identify the specific direction you are looking along e.g. [100] or [111]. It is then possible to calculate its relative direction to a reference frame depending on where the crossing is projected on the screen. All this is normally done by a computer
12
that can make one measurement in a fraction of a second. The SEM is then used to scan the sample making measurements in a grid pattern. The result can be illustrated in several ways, either by a pole figure using stereographic projection, an inverse pole figure or Euler space.
2.2.5 Parameters that Affect Texture and Anisotropy
The reason for deformation texture is crystal rotation. One way to explain the formation of texture is to consider a single crystal which deforms in only one slip system. Slip will occur together with a rotation since the crystal stays together and the points were the force is applied is not moved sideways. When the material is stretched the angle between the pull direction and the slip direction will decrease. When the material is compressed the opposite will occur [11], see figure 12.
Figure 12. Illustration of crystal rotation for a single crystal deformed in only one slip system.
An FCC material has 12 different slip systems consisting of a slip plane and a slip direction. The slip plane is normally the most dense plane in the structure, in the case of FCC a {111} plane. There are four different {111} planes, within each plane, three independent slip directions of type <110>. The system activated first is the one with highest resolved shear stress R given by the equation:
R cos cos
Equation 5where is the applied stress, is the angle between the slip and stress direction and is the angle between the slip plane normal and stress direction, see figure 13 [6].
13
Figure 13. Geometrical relationships used for calculating the resolved shear stress.
The factor coscos is normally referred to as de Schmid factor. The largest value of the Schmid factor, which gives the highest resolved shear stress, is 0.5. This happens when both angles are 45 degrees. All orientations have their own Schmid factor as can be seen in figure 14. The higher the Schmid factor is the higher will the resolved shear stress be in the most favored slip system. In a polycrystal it is likely that the deformation will start in a grain with a high Schmid factor. During the deformation the crystal will turn and the Schmid factor will change.
Figure 14. Inverse pole figure showing the Schmid factor for different crystal orientations.
The crystal rotation and resolved shear stress are not a complete explanation of the texture formation but a good start. Other factors have large impact on the texture, like what deformation mechanism that is active during the deformation.
When an FCC material is tensile tested the slip directions are turned towards the stress direction until two slip systems get equally beneficial. One direction where two slip systems have the same Schmid factor is the <112> direction. What happens then is a deviation into bands were some turn to a <111> direction and some to a <100> direction. The amount of the different orientations depends on the materials stacking fault energy. Low SFE gives more <001> type texture. When the SFE
14
decreases the amount of twins rises, twinning shears the <111> oriented crystals to a <115> direction from where they are turned to a <001> orientation [11].
The mechanical properties of austenitic stainless steels, particularly strain hardening is depending on the SFE. When lowering the SFE the deformation mechanism will change from slip in perfect crystals to slip in partials. With further decreasing SFE mechanical twinning will be activated, followed by transformation to ε -martensite and/or α’-martensite. It is important to know that regardless of deformation mechanism the greater part of plastic slip is accommodated through slip of dislocations [14].
The composition can affect the deformation mechanism by changing the SFE. Attempts have been made to create an equation to be able to calculate the SFE from the composition. Schramm and Reed [15] constructed an equation for commercial-grade austenitic Fe-Cr-Ni, Fe-Mn-Ni, and Fe-Cr-Ni-Mn steels. However it has later been concluded that no simple and universally valid composition equations exist for the SFE [16]
Temperature can affect the deformation mechanism since it changes the SFE [21]. The influence of temperature on texture formation will however not be included in this thesis.
2.3 Manufacture of OCTG
As mentioned earlier the OCTG have very high strength requirements and they are used in highly corrosive environments. Only stainless steels with exceptionally good resistance are suitable for OCTG applications. Due to the strength requirements cold working is necessary making austenitic and duplex stainless steels the most suitable alternative due to their good properties when cold worked.
2.3.1 Methods for Manufacture of OCTG
The cold pilgering process has many advantages over other tube processes such as cold draw. One advantage is that it allows much larger deformation rates and it gives a good tolerance for
dimensions. The pilgering is performed by two rolls, or dies that roll over the tube and reshaping the outside diameter. Each die has a groove were the diameter of the working section is shifting from large to small to fit the tube before and after pilgering. Inside the tube there is a mandrel that supports the tube and causes the wall reduction and shapes the inside diameter, see figure 15. The shape of the mandrel is as important as for the dies since the space between them forms the tube during the pilgering. The dies are supported by the saddle, during the pilgering the saddle is moving back and forth making the dies roll over the tube. To reduce the material the tube is fed into the tooling by controlled increments. The feeding is usually adjustable to allow for variations in material and dimensions of the tube. For every fed increment the tooling rolls over the tube and reduces the material, this step is defined a stroke. The feeding increment usually occurs when the dies are in their entry position. The entry position is one of the end positions of the movement of the saddle where the tube enters the tooling. There are machines that use double end feed where the feeding occurs at both end positions of the saddle. In order to maintain roundness of the tube both the tube and the mandrel are turned during the feeding [17].
15
Starting position
Position after one stroke
Mandrel Billet, extruded Tube
(
Finished Cold pilgered tube Roll groove
Mandrel bar
Figure 15. Illustration of cold pilgering.
When the tube is pilgered the reduction in wall thickness is not necessary the same as reduction in diameter. The relation between the reduction in wall thickness and diameter is known as the Q- factor according to
in out in in
out in
d d d t
t factor t
Q
Equation 6
where t is the wall thickness of the extruded tube, in tout is the wall thickness of the pilgered tube, d is the outside diameter of the extruded tube and in doutis the outside diameter of the pilgered tube. The Q-factor affects the surface finish and may be altered by changing the tool design [17]. It is also possible to divide the pilgering in small intervals and then calculate the Q-factor for each small interval to get more like a instant value for each point of the pilgering.
Sandvik AB uses the mean value of the inside and outside diameter when calculating the Q-factor instead of the outside diameter of the tube.
The pilgering is a multi directional deformation process. When the dies roll over the tube a small wave of material is pushed in front of the tool, this zone is called the squeeze zone. When the material gets in between the die and mandrel it is in the forge zone, see figure 16. During the pilgering the tube gets an elliptic shape, since it is pressed from two directions. This is good in one aspect because it hinders the tube from getting stuck on the mandrel. The width of the contact area between the tube and the die relative to the thickness of the wall determines how homogeneous the deformation is. The most homogeneous deformation occurs when the contact length equals two times the wall thickness (this corresponds to the criteria L/hm = 1 used for rolling processes where L is the contact length and hm is the mean thickness). A small contact area relative to the wall thickness causes inhomogeneous deformation were most deformation occurs close to the surface (L/hm < 1).
16
On the other hand if the contact area is much larger than the width of the wall, friction will occur (L/hm > 1). The friction causes inhomogeneous deformation.
Figure 16. Illustration of the deformation of the tube during pilgering. The image is exaggerated.
The alternative to cold pilgering is cold drawing were the tube is pulled once or twice through a tool.
The drawing of OCTG is usually done by a draw bench that can draw the entire tube in one stretch.
For other applications the drawing is performed in steps were a new grip is made for every step.
There are several types of tools and ways of performing the drawing, some are illustrated in figure 17. Most of the methods are one step processes except rod drawing where the second step is removing the rod. Cold drawing does not enable as large reductions as cold pilgering and is a much faster process.
Figure 17. Principle of drawing processes, a) fixed plug draw, b) floating plug draw, c) rod drawing, ) sink drawing and e) expanding
2.3.2 Plastic Deformation Mechanisms and Anisotropy during Cold Working
Cold drawn tubes reach about the same strength as a cold pilgered tubes even though the reduction is lower. When performing the drawing the deformation in the tube is more or less in the same direction throughout the entire process. The deformation can be compared with a one step rolling operation. This assumption is backed by Park and Lee [18] that studied texture in an FCC material during cold drawing. The drawing was made using plug drawing and the conclusion was that the texture was very similar to rolling texture.
During pilgering the deformation is very complex and very few investigations have been made on the texture on pilgered tubes, especially not on austenitic steels. When performing pilgering the
deformation is formed with small increments, one for each stroke. The deformation is shifting direction since the rolls roll over the tube in both directions and the tube is turned during pilgering
17
changing the angle of deformation. Since the deformation shifts direction so does the slip plane allowing large deformations. The Q-factor can have influence on the texture.
2.4 Previous Work
This section presents a small selection of scientific articles that gives a small hint of what can be expected by this thesis. The results will be compared with these articles in order to confirm them.
Karaman et al. [19] studied the influence of nitrogen on the stress-strain behavior of low stacking fault energy AISI 316L austenitic stainless steel. They used single crystals and performed tensile tests at room temperature for different crystallographic directions. Transmission electron microscopy was used to study the twinning. The test showed twin formation at strains as low as 3 % for some
directions and that twinning is a strong barrier to dislocation motion by confining the mean free path of the dislocations. When adding 0.4 % nitrogen to the alloy the critical resolved shear stress increase due to higher lattice friction and short range order formation. This leads to a change in deformation mechanisms and deviations from Schmid Law. Twinning is suppressed in all directions which
decrease the strain hardening coefficient. The change in deformations mechanisms can be explained by the increase in stacking fault energy caused by the addition of nitrogen.
Lee et al. [20] studied the deformation twinning in high-nitrogen austenitic stainless steels. They used a steel with composition: 17.94Cr-18.60Mn-2.09Mo-0.89N-0.04C-balance Fe (wt %). With in-situ EBSD they showed that grains oriented in the <001> or <111> orientation kept their original
orientation or rotated along the <001>-<111> boundary line towards the <112> orientation. Grains with other orientations (in particular near <110> orientation) rotated towards the primary slip direction until reaching the <001>-<111> boundary line. They confirmed that the orientation change showed the same trend as for FCC single crystals. They also used TEM to study the twin formation showing stacking faults and deformation twinning in the high-strain regime while the microstructure in the low strain region consisted of planar dislocation arrays. The twinning showed strong
orientation dependence. Grains oriented with <100>, <110> and <111> direction parallel to the tensile direction showed no deformation twinning, one activated twinning system and primary and conjugate systems cooperated, respectively.
Dumay et al. [21] used thermochemical modeling for calculation of how the stacking fault energy is affected by addition elements in austenitic Fe-Mn-C stainless steel. Their material (Fe22Mn0.6C) show high formation of microtwins for SFE around 20 mJ/m3. When the SFE drops below 18 mJ/m3
microtwin formation are replaced by HCP ε-martnsite and when it drops below 12 mJ/m3
α´-martensite is formed. When the SFE is increased the amount of twinning decreases and other deformation mechanisms take place. Twinning forms due to stacking-faults in the FCC structure extending in parallel adjacent planes. When the twinning extends to every two planes it leads to the formation of ε -martensite. They also showed that copper increases the stacking fault energy.
Simmons [7] wrote a review of high-nitrogen alloying of stainless steels. It is stated that austenitic stainless steels typically have low stacking fault energy. Their deformation structure is characterized by planar dislocation arrays while high SFE materials generally form cell-type dislocation structure.
The planar dislocations reduce the cross-slip leading to slip-band formation and enhanced strain hardening.
18
3. Test Material
The thesis is divided in two parts, the first and most extensive concerns the texture formation during plastic deformation. The purpose of the examination is to learn what influence the alloying elements have on the crystal rotation for austenitic stainless steels that are suitable for OCTG applications. The reason for this is that crystal rotation is one source of anisotropy, which is a problem for cold
pilgered tubes. The focus lies on the elements molybdenum, nitrogen and copper. Austenitic steels have been selected in pairs where there is a difference in each of these elements according to table 1. The examination is expanded with examinations of synergy effects and the influence of grain size.
Table 1. Steel grades used for texture evolution study (3R69, Sanicro 30 and 254 SMO are not used for OCTG applications by Sandvik).
Influence Steel grade Heat Shape
Molybdenum
{
Sanicro 28 high N 522031 7” tubeSanicro 29 522137 7” tube
Nitrogen
{
3R69 713413 Bar, diameter 15 mm3R60 498916 Tube
Copper
{
Sanicro 30 772082 Bar, diameter 22 and 32 mmSanicro 41 463664 Bar, diameter 22 mm
Nitrogen
{
Sanicro 28 520553 7” tubeSanicro 28 high N 522031 7” tube
Synergy 254 SMO 474816 Bar, 15 mm
Grain size Sanicro 30 772082 Bar, diameter 22 and 32 mm
The other part of the thesis concerns evaluation of the tool design used for the cold pilgering. For this evaluation three different 7” tubes are examined all from the same heat but pilgered with different tool designs. The steel grade used is Sanicro 29 heat 522148.
3.1 Steel Grades
All steels used in this thesis are austenitic stainless steels with face centered cubic crystal structure.
The eight different steel grades have been chosen for their chemical composition more than for their suitable applications. The chemical composition is very important to enable the possibility to
examine the influence of specific alloying elements, however some of the steel grades are very well suited for OCTG applications such as Sanicro 28 and 29.
3.1.1 Sanicro 28
Sanicro 28 is a multi-purpose austenitic stainless steel well suited for OCTG applications and is attended for service in highly corrosive conditions. For chemical composition see table 2. In this investigation two versions of Sanicro 28 is used one with slightly higher nitrogen content. The nitrogen contents measured are 0.047 and 0.091 respectively.
19
Table 2. Chemical composition (nominal %) of Sanicro 28 according to datasheet at Sandvik AB webpage [22].
C Si Mn P S Cr Ni Mo Cu
< 0.020 < 0.6 < 2.0 < 0.025 < 0.015 27 31 3.5 1.0
3.1.2 Sanicro 29
Sanicro 29 is suited for OCTG applications in particularly corrosive conditions. The alloy is developed for improved localized corrosion resistance in environments with high chloride concentrations, CO2
and H2S at high temperatures. It is highly alloyed, see table 3, to be able to have very good corrosion resistance.
Table 3. Chemical composition (nominal %) of Sanicro 29 according to datasheet at Sandvik AB webpage [22].
C Si Mn P S Cr Ni Mo Cu
< 0.020 < 1.0 < 2.5 < 0.025 < 0.015 27 31.5 4.4 1.0
3.1.3 Sanicro 30
Sanicro 30 is used for tubing in heat exchangers and steam generators in nuclear stations for temperatures up to about 550°C were it is important with good resistance to stress corrosion cracking and intergranular corrosion. Its chemical composition can be seen in table 4.
Table 4. Chemical composition (nominal %) of Sanicro 30 according to datasheet at Sandvik AB webpage [22].
C Si Mn P S Cr Ni Cu Ti Al
< 0.030 0.5 0.6 < 0.020 < 0.015 20 32 < 0.10 0.5 0.3
3.1.4 Sanicro 41
Sanicro 41 is an austenitic, corrosion resistant Ni-Fe-Cr alloy. It is used for heat exchanger tubing and for tubing and casing in oil and gas production, especially suited for use in environments with high concentrations of hydrogen sulphide and chloride. See table 5 for chemical composition.
Table 5. Chemical composition (nominal %) of Sanicro 41 according to datasheet at Sandvik AB webpage [22].
C Si Mn P S Cr Ni Cu Ti Mo
< 0.030 < 0.5 0.8 < 0.025 < 0.015 20 38.2 1.7 0.7 2.6
3.1.5 3R60
3R60 is a multi-purpose austenitic steel where there is need of good corrosion resistance like heat exchangers, condensers, pipelines, cooling and heating coils in the chemical, petrochemical, pulp and paper and food industries. Its chemical composition can be seen in table 6.
20
Table 6. Chemical composition (nominal %) of 3R60 according to datasheet at Sandvik AB webpage [22].
C Si Mn P S Cr Ni Mo
< 0.030 0.4 1.7 < 0.040 < 0.015 17.5 13 2.6
3.1.6 3R69
3R69 is similar to 3R60 but has a higher content of nitrogen, see table 7.
Table 7. Chemical composition (nominal %) of 3R69 according to datasheet at Sandvik AB webpage [22].
C Si Mn P S Cr Ni Mo N
< 0.030 0.4 1.7 < 0.030 < 0.015 17.5 13 2.6 0.18
3.1.7 254 SMO
254 SMO is a austenitic steel approved for seamless pipe in boiler and pressure vessel applications and for sulfide stress cracking resistant material for oil field equipment. Its chemical composition can be seen in table 8.
Table 8. Chemical composition (nominal %) of 254 SMO according to datasheet at Sandvik AB webpage [22].
C Si Mn P S Cr Ni Cu Mo N
< 0.020 < 0.80 < 1.00 < 0.030 < 0.010 20 18 0.7 6.1 0.20
3.2 Heat Treatment and Grain Size
In order to get comparable results it is important that the materials have the same starting situation with respect to residual stress and grain size. The ideal starting material would be a completely relaxed material with uniform grain size and no other phases than austenite. The best way to get close to this situation is to heat treat the material to get it recrystalized. Most of the materials were heat treated in order to get a desired structure with similar grain size and undeformed grains, see table 9.
Table 9. Heat treatments of the steel grades, all materials were quenched in water.
Steel grade Temperature (°C) Hold time (min)
Sanicro 28 1125 3
Sanicro 29 1180 3
Sanicro 30 1050 25
3R60 1100 20
254 SMO 1180 5
To get larger grains in Sanicro 30 it was held at 1050 °C for 90 minutes and then quenched in water.
To further increase the grain size it was held at 1100 °C for 60 minutes and then quenched in water.
The measured grain sizes after the heat treatments will be presented in the result section.
21
3.3 Test Samples
The test samples were in shape of tensile testing specimen with diameter 7.0 ± 0.4 mm and active length 70 mm, denotation DR7C70, see appendix I for geometries and drawing. All test samples were taken in the lengthwise direction of the rod or tube, see figure 18. The specimens were fabricated by turning in a CNC operated lathe at Sandvik AB in Sandviken.
Figure 18. Illustration of how the tensile specimens were taken out from tubes and solid bars and which parts of the specimen that were used for evaluation.
After straining, samples for LOM, EBSD and hardness testing were taken out from each tensile specimen.
For the evaluation of tool designs Rockwell hardness measurements were made. The test samples for hardness measurements were in shape of cut out rings, all were cut out close to the middle of the tubes. The rings were cut out using a lathe and were then ground to get smooth and parallel sides.
The samples for EBSD texture measurements were also cut out close to the middle of the tube according to figure 19.
Figure 19. Illustration of where the samples were cut out for evaluation of tool designs. The tube length is not scaled to its diameter.
22
4 Experimental
This section will describe how the measurements were performed.
4.1 Sample Straining
In ordered to be able to investigate how the chemical composition affects the texture formation the specimen need to be deformed in a controlled way. The easiest way to do this is to use tensile testing equipment. A test pull was performed for all materials in order to see at which strain the necking begins. All deformation up to the point of maximum stress were the necking begins is uniform throughout the narrow region of the tensile specimen [6]. This is of course valid in a macroscopic perspective due to that strain hardening is larger than area decrease and requires that the material is homogenous. It is important for the investigation since several samples were taken out from each specimen. Each steel grade was deformed to five different strains that were evenly distributed in the interval up to necking, see table 10. All tensile specimens were measured before and after
deformation and the area reduction of the cross section were calculated. The diameter
measurements were performed with a digital micrometer from Mitutoyo and the area was marked so that the EBSD sample was taken from the same place if there would be any deviations along the active part of the specimen. All straining was performed with a Roell & Korthaus tensile testing machine controlled by the program Cyclic by Inersjö Systems AB. The stain rate was 1 mm/min.
Table 10. Strain and area reduction for the specimens.
Sanicro 28 Strain (mm) 7 14 21 28 35
Area reduction (%) 6.7 13.2 19.0 24.4 30.3 Sanicro 28
high N
Strain (mm) 7 14 21 28 35
Area reduction (%) 7.6 14.0 19.7 25.3 30.9
Sanicro 29 Strain (mm) 7 14 21 28 35
Area reduction (%) 7.3 13.1 18.7 23.9 28.6
Sanicro 30 Strain (mm) 6 12 18 24 30
Area reduction (%) 5.9 11.4 16.2 21.3 25.7
Sanicro 41 Strain (mm) 7 14 21 28 35
Area reduction (%) 7.1 13.3 18.5 23.7 28.5
3R60 Strain (mm) 7 14 21 28 35
Area reduction (%) 6.7 12.8 18.9 24.3 29.8
3R69 Strain (mm) 3 7 11 15 19
Area reduction (%) 2.8 10.6 11.6 16.0 24.8
254 SMO Strain (mm) 7 14 21 28 35
Area reduction (%) 6.7 13.0 18.8 23.2 28.5 Sanicro 30
large grains
Strain (mm) 6 12 30
Area reduction (%) 5.8 10.9 25.4
23
4.2 EBSD
Electron back scatter diffraction was used to measure the texture. The technique is described in the theory section. The instrument used was a Zeiss 1540 EsB Cross beam FIB-SEM equipped with EBSD apparatus by Oxford Instruments. The middle part of each rod was carefully cut out not to damage the microstructure or texture using equipment by Struers. Measurements were performed at the transversal cross section of each rod. The samples were prepared for measurements by the following steps:
1. In-molded in 25 mm pellets of Struers Polyfast
2. Surface-grinding, removing at least 1 mm to be sure to eliminate any plastic deformation from the cutting
3. 6μ grinding 4. 3μ grinding
5. OPS-polishing with hydrogen peroxide (100:15 OPS-solution and hydrogen peroxide) 6. The surface was carefully cleaned using ethanol soaked cotton pads
All sample preparation was performed using equipment by Struers.
The measurements were made at half radius of the sample with acceleration 20 kV, step size 1.8 µm in a raster of 1173x244 points, se figure 20. The parameters were chosen with respect of the texture measurement only. During the measurements the sample was tilted 70 degrees and the indexing was made with the HKL Channel 5 program Flamenco by Oxford Instruments.
Figure 20. Illustration of were on the sample the measurements were made.
EBSD measurements were also carried out on the Sanicro 29 tubes pilgered with different tool designs. Here the measurement parameters were acceleration 20 kV, step size 1.8 µm and the raster were 500x500 points. All measurements were made in the transversal direction but on three
different locations, close to the outside surface, in the middle of the wall and close to the inside surface for the different tool designs.
24
4.3 LOM
The microstructure was examined with light optical microscopy (LOM). Optical lenses are used to give a magnified image of the specimen. Visible light is used which makes it easy to use because it gives a direct picture that can be captured by a normal camera. The technique has limitations in a low focus depth and resolution of only about 2 μm due to the wave length of visible light.
The samples were prepared by grinding and polishing down to 3 μm and etching. The etching was performed in V2A or electrolytically in oxalic acid (voltage 3V). The V2A etch is consisting of 50 ml hydrochloric acid, 50 ml water, 5 ml nitric acid and 2 drops of Rodine. The V2A etch was heated to about 60°C before etching.
4.4 Vickers Hardness
When measuring Vickers hardness a small square-based diamond pyramid is pressed on the surface with loads normally between 1 and 10 kg, sometimes lower but is then referred to as microhardness.
The pyramid has an angle of 136° and Vickers hardness is then calculated with the formula:
/
2854 .
1
P dHV Equation 7
where P is the load in kg and d is the average diameter of the increment [6].
In this thesis HV5 were used (load 5 kg) to study deformation hardening during deformation. The samples where baked in Bakelite and ground down to 3 µm. Each sample was measured with three indents on both transversal and lengthwise cuts.
4.5 Rockwell C Hardness
When measuring Rockwell C a diamond cone is pressed towards the surface, the hardness number is determined by the difference in penetration depth between an initial minor load and a following major load. The cone angle is 120°, the minor load is 10 kg and the major is 150 kg [6].
Measurements were performed at Sandvik AB in Sandviken. All measurements were made on a transversal cut of the tube. Increments were made at three positions, 2.54 mm from the inner and outer edge and in the middle of the wall. The measurements were made every ten degrees around the tube.
25
5 Results and Discussion
All results will be presented in this section together with a discussion.
5.1 Microstructure of Reference Samples (LOM)
The steel grades 3R69 and Sanicro 41 were not heat treated due to that their microstructure appeared undeformed and grain growth wanted to be avoided. The microstructure is shown in figure 21. 3R69 had large amounts of twinning and a large variation in grain size while Sanicro 41 had more uniform grain size and a small amount of twinning.
Figure 21. Microstructure of a) 3R69 and b) Sanicro 41.
The source materials for Sanicro 28 and 29 were cold pilgered tubes with large deformations. The deformed and annealed structure of Sanicro 29 can be seen in figure 22. The reference sample of Sanicro 29 had a microstructure with twins and no other obvious phases than austenite. The reference samples of the two Sanicro 28 grades had very similar structure to Sanicro 29 after the heat treatment. The only difference was that Sanicro 29 had a slightly larger grain size.
Figure 22. Microstructure of Sanicro 29, a) as received and b) after heat treatment.
Sanicro 30 had much smaller grains than the other materials before the heat treatment. After heat treatment the structure showed an amount of twinning similar to Sanicro 29 but the grain size was still smaller than both Sanicro 28 and Sanicro 29. Four specimen of Sanicro 30 were heat treated to get larger grains. This heat treatment resulted in inhomogeneous grain growth with some areas with very large grains and some with grain sizes similar to Sanicro 28, see figure 23.
26
Figure 23. Sanicro 30 heat treated to get large grains showing inhomogeneous grain growth, a) area with large grains and b) area with smaller grains.
The microstructure was also evaluated using EBSD which will be presented later in this report including measured grain size and amount of twinning.
5.2 Microstructure After Deformation for Sanicro 29 (LOM)
The deformed structure is shown for one steel grade. Sanicro 29 has similar microstructure as several other steel grades in this investigation and its deformation can be regarded as typical for these steels. Figure 24 shows the undeformed material, where it is possible to see the distribution of twins.
There is no difference in the structure obtained from the transversal compared to the lengthwise cut.
Figure 24. Reference sample of Sanicro 29, a) lengthwise and b) transversal.
Figure 25 shows the structure at maximum deformation, just before necking. The lengthwise structure is clearly deformed and stretched in the strain direction. Both in the lengthwise and transversal structures slip bands are clearly formed. The transversal structure seems to have smaller grain size in the deformed structure compared to the reference sample. This is due to the stretching of the grains, the section area of the grains will decrease as the section area of the tensile sample decrease during deformation.
