WO2018002328A1

WO2018002328A1 - A new process for manufacturing an austenitic alloy

Info

Publication number: WO2018002328A1
Application number: PCT/EP2017/066329
Authority: WO
Inventors: Raghuveer GADDAM; Guocai Chai; Mikael GREHK; Karin ANTONSSON; Ardeshir GOLPAYEGANI
Original assignee: Sandvik Intellectual Property Ab
Priority date: 2016-07-01
Filing date: 2017-06-30
Publication date: 2018-01-04

Abstract

The present disclosure relates to a new process for manufacturing an object comprising an austenitic alloy. The process combines a cryogenic treatment with a heat treatment whereby the obtained object will posse excellent properties, especially in regard of strength and ductility.

Description

A new process for manufacturing an austenitic alloy Technical field

The present disclosure relates to a new process for manufacturing an object comprising an austenitic alloy. The process combines a cryogenic treatment with a heat treatment whereby the obtained object will have excellent properties, especially a combination of excellent strength and ductility.

Background

Austenitic stainless steels and austenitic nickel-based alloys form an important group of alloys. Both alloys are widely used in many different applications due to their excellent corrosion resistance, ductility and good strength. There are various ways of

manufacturing such alloys but during the strengthening step there is a problem of unwanted reduction of the ductility. However, lately, the introduction of nano-twins in some of these alloys has proven to be an effective way to obtain materials with high strength and high ductility. As the microstructure of especially stainless steels will depend on the composition and on the manufacturing process, all alloys are not susceptible to such processing. It is known from US 4, 161 ,415 that the strength of an alloy may be improved by enhancing the concentration of martensite. The concentration of martensite is enhanced by altering the elementary concentrations or by lowering the temperature to cryogenic temperature (CT). However, as this document relates to martensitic alloys, the transformation process involved is different from an austenitic alloy.

Hence, there is a need for new processes which can be used for a wide variety of austenitic alloys. Furthermore, there is a need for new processes which will be able to increase more than one property of the austenitic alloys. Summary

Hence, an aspect of the present disclosure is to provide a process which will make it possible to improve more than one property of an austenitic alloy, especially the strength and the ductility. Therefore, the present disclosure provides a process for manufacturing a product containing an austenitic alloy, the austenitic alloy comprising in weight (wt%):

C max 0.06;

Si max 2.0;

Mn max 6;

P max 0.04;

S max 0.04;

Cr 15 to 30;

Ni more than 14 to 75;

Mo max 8; W max 8; and wherein Mo+1/2 W max 8;

N max 0.4;

Cu max 2;

Nb max 0.5;

Ti max 1 ;

balance Fe and normal occurring impurities. The process of the present disclosure combines a cryogenic treatment with a heat treatment whereby the obtained object will have excellent strength and ductility.

Brief description of the figures

Figure 1 discloses a graph showing the tensile strength of the four alloys #1

- #4 of Example 1 , a graph measured at both RT (room

temperature) and CT (cryogenic temperature);

Figures 2a to 2b discloses a schematic illustration of the tensile properties measured at room temperature for few of the alloy 2, Figure 2(a) shows the yield strength and tensile strength and figure 2(b) shows the ductility;

Figure 3 a to 3b discloses a schematic illustration of the tensile properties measured at room temperature for few of the alloy 4, Figure 3(a) shows the yield strength and tensile strength and figure 3(b) shows the

ductility;

Figure 4a to 4b discloses a schematic illustration of the tensile properties measured at room temperature for few of the alloy 7, Figure 4(a) shows the yield strength and tensile strength and figure 4(b) shows the ductility;

Figure 5 discloses the hardness as function of: temperature for a heat

treatment time of 1 h for the alloys of Example 3.

Detailed description

The present disclosure relates to a process for manufacturing a product comprising the steps of:

a. providing an object containing an austenitic alloy having the following composition (in weight%):

C max 0.06;

Si max 2.0;

Mn max 6;

P max 0.04;

S max 0.04;

Cr 15 to 32;

Ni more than 14 to 75;

Mo max 8; W max 8; and wherein Mo+1/2 W max 8;

N max 0.4;

Cu max 2;

Nb max 0.5;

Ti max 1 ;

balance Fe and impurities b. bringing the object to a temperature below 0°C; c. plastically deforming the object in this temperature; d. heat treating the deformed object in a temperature of from about 400 to about 600 °C during a predetermined period of time. During the plastic deformation of the austenitic alloy at low temperatures, nanotwins are formed in micro structure. Furthermore, no subgrain was observed. . This will increase the strength of the austenitic alloy. Further, due to the nanotwins, the plastic deformation is also accompanied by dislocation. Additionally, it has surprisingly been found that even though a heat treatment step is being performed on the plastically deformed austenitic alloy, which normally would lower the strength, the combined mechanical properties, i.e. the strength and the ductility, are improved. Thus, the present method will provide for that the final product will have a combination of excellent mechanical properties. Without being bound to any theory, it is believed that the nanotwins from the cryogenic deformation may have an impact on the mechanical properties even after the heat treatment.

The predetermined time of the heat treatment will depend on the size and the thickness of object to be heat treated and is performed until the desired ductility and strength have been obtained.

According to one embodiment of the method as defined hereinabove or hereinafter, the the object is brought to a temperature below -50°C before the plastic deformation is imparted to the material, such as a temperature below -75°C, such as below -100°C, such as below -150°C such as below or equal to -196 °C.

According to one embodiment of the present method, the deformation is performed by drawing.

According to the method as defined hereinabove or hereinafter, the heat treatment performed in a temperature of from 400 to 550 °C, such as 400 to 500 °C. According to one embodiment of the method as defined hereinabove or hereinafter, the heat treated object is cooled directly after the heat treatment.

According to one embodiment, the object is heated to room temperature before the heat treatment step.

According to one embodiment, the plastic deformation imparted to the austenitic alloy is at of at least or equal to 50%,. According to a further embodiment, a plastic deformation is imparted to the austenitic alloy of at least or equal to 70%.

The obtained product may be a tube, a pipe, a strip, a wire, a bar or a plate. According to one embodiment, the obtained product is a tube or a bar or a strip or a wire The general dependence of the different components of austenitic alloy is discussed below, however, the functions described of the elements may not be complete, the elements may also have other functions in the alloy not mentioned here. Further, the compositional ranges that delimit the austenitic alloy according to the disclosure are specified in weight% (wt%). According to one embodiment, the present austenitic alloy consists of the elements in the ranges disclosed below.

Carbon (C) is an austenite stabilizing element but most austenitic alloys have low carbon contents, max 0.08 wt%. The austenitic alloy according the disclosure has an even lower carbon content level, i.e. max 0.06 wt%. This low carbon content will reduce the formation of chromium carbides that otherwise would result in an increased risk of intergranular corrosion attacks. Low carbon content may also improve the weldability.

Silicon (Si) is used as a deoxidising element in the melting of austenitic alloy. However, too much silicon will be detrimental to weldability. The austenitic alloy according to the disclosure has therefore a Si-content of max 2.0 wt%. Manganese (Mn), is effective to improve the hot workability. However, the amount of Mn is limited to max 6 wt%, in order to control the ductility and toughness of the austenitic alloy at room temperature. According to one embodiment, the austenitic alloy according to the disclosure has a Mn-content of max 4 wt%.

Chromium (Cr) is a ferrite stabilizing element. Also, by increasing the Cr content, the corrosion resistance increases. However, a too high Cr content may increase the risk of formation of intermetallic phase, such as sigma phase. Additionally, in order for an austenitic alloy to have corrosion resistance, the content of Cr must be at least 11 wt%. The austenitic alloy according to the disclosure has therefore a Cr-content of from 15-30 wt%.

Nickel (Ni) is an austenite stabilizing element. A high nickel content will provide a stable austenitic microstructure and suppress the formation of intermetallic phases like the sigma phase. However, the present disclosure has shown (see Example 1), that in order to achieve plastic deformation by the formation of nanotwins (twinning), the content of Ni must higher than about 14 wt%. According to one embodiment, the Ni content is least 21 wt% or higher. As Ni is an expensive element, the upper limit is set to 75 wt%.

According to one embodiment, the austenitic alloy of the present disclosure is a nickel- based austenitic alloy. According to one embodiment, the Ni content is at least 21 wt% to 70 wt%.

Molybdenum (Mo) and Tungsten (W) are both ferrite stabilizing elements. Addition of Mo and/or W greatly improves the general corrosion resistance of an austenitic alloy. However, too high amounts of Mo and/or W will promote the formation of sigma-phase. The austenitic alloy according to the disclosure has therefore a Mo-content of max 8 wt% or a W content of max 8 wt% or a Mo+W/2 content of max 8 wt%. According to one embodiment of the present disclosure, the content of these elements, either alone or in combination is max 4 wt%. Sulfur (S) influences the corrosion resistance negatively by the formation sulfides.

Sulphur also decreases hot ductility during hot working. Therefore, the content of S should be restricted to max. 0.04 wt%. Phosphorus (P) is an impurity. If present in amounts greater than approximately 0.04 wt%, it may result in adverse effects on e.g. hot ductility, weldability and corrosion resistance. The amount of P in the alloy should therefore be restricted to max 0.04 wt%.

Copper (Cu), the addition of copper may improve both the strength and the resistance to corrosion in some environments, such as in sulphuric acid. A too high amount of Cu may however lead to a decrease of ductility and toughness. The austenitic alloy has therefore a Cu-content of max 2.0 wt%.

Nitrogen (N) is a strong austenite stabilizing element. The addition of nitrogen may improve the strength and corrosion resistance of austenitic alloy as well as the

weldability. N will also reduce the tendency for formation of sigma-phase. The austenitic alloy according to the disclosure therfore has a N-content of max 0.4 wt%.

Titanium (Ti) and Niobium (Nb) may be added for the purpose of improving the creep rupture strength through precipitation. However, an excessive amount of Ti and/or Nb may decrease the weldability, the maximium of Ti is therefore 1 wt% and the maximum content of Nb is therefore 0.5 wt%.

When the terms "max" or "maximum" are used, the skilled person knows that the lower limit of the range is 0 wt% unless another number is specifically stated. Hence, for C, Si, Mn, Cu, S, P, Mo, W, N, Nb and Ti the lower limit is 0 wt%, as they are optional components.

Additionally, other elements may optionally be added to the austenitic alloy as defined hereinabove or hereinafter during the manufacturing process in order to improve for example the processability, the hot workability, the machinability etc. Examples, but not limiting, of such elements are Hf, Ca, Al, Ba, V, Ce, Mg and B. If added, these elements may be added in an amount of max 1.0 wt% in total.

The balance in the austenitic alloy defined hereinabove or hereinafter is Fe and unavoidable impurities. Examples of unavoidable impurities are elements and compounds which have not been added on purpose, but cannot be fully avoided as they normally occur as impurities in e.g. the material used for manufacturing the austenitic alloys.

The present disclosure is further illustrated by the following non-limiting examples:

EXAMPLES Example 1

Four high purity alloys (i.e. alloys having low content of other alloying elements other than Fe, Cr, and Ni) were melted and casted as 170-270 kg ingots in a vacuum induction melting furnace (VIM) and then quenched in water. Forging of the as-cast ingots were performed to dimension 136x56 mm². Furnace temperature at forging was 1250 °C. After the forging operation, the material was quench-annealed which involved the steps:

heating to a temperature of 1200 °C with a holding time of 30 min, followed by water quenching. Hot rolling was performed in order to reach the final thickness of 15 mm. The furnace temperature during hot rolling was 1210 °C. After the final rolling pass, the material was quench-annealed again for 20 min at 1210 °C before quenching in water. The nitrogen level was <0.008 wt%, carbon < 0.009 wt% and the total amount of trace elements was below 0.15 wt%. For a complete listing of sample compositions, see Table 1.

As can be seen from Table 1 , these grades of stainless have chromium (18.6% to 18.8%) and nickel (11.7 to 31.4%) as their major alloying additions. Table 1. Chemical composition in wt% of the

four high purity alloys (#l-#4).

Figure 1 shows the influence of temperature RT (room temperature) and CT (cryogenic temperature) on the tensile properties of these four high purity alloys. At RT, the curves are collected and reaches an UTS (ultimate tensile strength) value of around 440-480 MPa. As can be seen from the figure, the situation changes dramatically at CT. The samples #1 and #2 show a very high work hardening potential. The change in tensile properties is accompanied by a change in magnetic properties and sample #1 and #2 become magnetic after deformation, i.e. they will become martensitic. This is due to the fact that these two samples will have transformation induced plasticity, i.e. the plastic deformation will induce a phase deformation from austenite to martensite in these two alloys. For the alloy #3 and #4 the situation is different. Both these alloys will remain paramagnetic after the plastic deformation and the deformation can be described as twining induced plasticity and the improved mechanical properties.

Example 2

In this example, the influence of plastic deformation at cryogenic temperature together with heat-treatment in the temperature range of from 400-800 °C for eight austenitic alloys having a concentration interval is investigated: Table 2a The concentration interval (in wt%)

The alloys were obtained from AB Sandvik Materials Technology in solution annealed condition. Table 2B shows the chemical analysis of the alloys investigated. These alloys were cryo-deformed to 70% of the fracture strain at -196°C. This was followed by the heat treatment for about 6 hours at 400, 600, and 800°C respectively and then the treated alloys were water quenched.

Table 3 provides the tensile properties at room temperature for different conditions of the alloys, i.e. in room temperature without any treatment, after deformation in cryogenic temperatures, after deformation in cryogenic temperature at 70% strain and after deformation in cryogenic temperature and then heat treated at different temperatures. . The tensile properties evaluated are yield strength (Rp0.2), tensile strength (Rm) and ductility expressed as total elongaion after fracture (A). It is noted that for the alloys 1-7 yield strength increased to about 20-30% and tensile strength (Rm) increased about 15- 22%. The increase in the properties is observed by comparing the properties obtained by performing the heat treatment after cryogenic temperature deformation and the properties observed by performing the heat treatment after deformation at room temperature (i.e. without cryogenic deformation). However, the ductility reduced by about 13-36%. Here it is observed that the tensile properties were not improved in alloy 8, which possessed higher amount of Ni.

Table 3. Tensile properties of the investigated austenitic alloys

Rpo.2 Rpi.o Rm A A50 Z

Alloy Sample condition

[MPa] [MPa] [MPa] [%] [%] [%]

Tested until failure at

331 378 709 60.8 45.4

1 RT 76

Tested until failure at

724 812 1293 69.2 57.3

-196°C 68

70% deformed at - 196°C + tested until 874 958 978 28.9 20.1 failure at RT 67

70% deformed at - 196°C + HT 400°C +

tested until failure at

RT 960 968 1002 30.1 20.2 68

70% deformed at - 196°C + HT 600°C +

tested until failure at

RT 862 882 985 32.6 23.3 65

70% deformed at - 196°C + HT 800°C +

tested until failure at

RT 695.5 762 973 26.2 20.8 32.5

Tested until failure at

2 271 316 642 64.4 44

RT 84

70% deformed at RT

+ tested until failure 791 792 828 28 - 79 at RT

70% deformed at RT

+ HT 400°C + tested 798 798 860 32 - 76 until failure at RT

70% deformed at RT

+ HT 600°C + tested 689 709 832 33 - 76 until failure at RT

Tested until failure at

573 652 1141 73 62

-196°C 75 70% deformed at - 196°C + tested until 848 974 990 30 14.9 failure at RT 65

70% deformed at - 196°C + HT 400°C +

tested until failure at

RT 987.5 987.5 996.5 25.4 16.2 70

70%) deformed at - 196°C + HT 600°C +

tested until failure at

RT 835.5 860.5 952.5 29.0 19.7 63

70%) deformed at - 196°C + HT 800°C +

tested until failure at

RT 601 668 858.5 22.9 19.0 26

Tested until failure at

284 323 660 58 45 RT 82

Tested until failure at

593 656 1136 76.6 61 -196°C 66

70%) deformed at - 196°C + tested until 863 988 997 30.8 18.7 failure at RT 67

70%) deformed at - 196°C + HT 400°C +

tested until failure at

RT 999 1001 1011.5 27.0 17.5 69

70%) deformed at - 196°C + HT 600°C +

tested until failure at

RT 856 875 968 29.9 20.6 66

70%o deformed at - 196°C + HT 800°C +

tested until failure at

RT 637.5 703 892 26.9 20.4 32

Tested until failure at

RT 227 260 536 53.2 44 78

70% deformed at RT

+ tested until failure 669 668 686 29 - 74 at RT

70% deformed at RT

+ HT 400°C + tested 690 691 723 28 - 72 until failure at RT 70% deformed at RT

+ HT 600°C + tested 621 642 733 30 - 71 until failure at RT

Tested until failure

at -196°C 339 389 864 91.6 82 72

70% deformed at - 196°C + tested until

failure at RT 856 916 920 21 7.8 65

70%) deformed at - 196°C + HT 400°C +

tested until failure at

RT 0 0 887 0.0 0.0 0

70%) deformed at - 196°C + HT 600°C +

tested until failure at

RT 809 827.5 869 22.8 14.7 55

70%) deformed at - 196°C + HT 800°C +

tested until failure at

RT 249.5 290 576.5 48.0 37.0 73

Tested until failure at

439 466 798 68.6 56 RT 77

Tested until failure at

870 940 1503 61.8 45 -196°C 58

70%) deformed at - 196°C + tested until 983 1037 1099 40.5 28.3 failure at RT 66

70%) deformed at - 196°C + HT 400°C +

tested until failure at

RT 1014 1012 1096 36.9 25.5 68

70%o deformed at - 196°C + HT 600°C +

tested until failure at

RT 813.5 824.5 951 34.4 24.7 64

70%) deformed at - 196°C + HT 800°C +

tested until failure at

RT 754 805.5 947.5 10.1 9.5 4

Tested until failure at

RT 211 243 540 53 46 77

70% deformed at RT

+ tested until failure 699 699 731 29 - 75 at RT 70% deformed at RT

+ HT 400°C + tested 720 722 767 28 - 74 until failure at RT

70% deformed at RT

+ HT 600°C + tested 633 653 759 33 - 74 until failure at RT

Tested until failure

at -196°C 368 425 859 88.6 69 75

70%) deformed at -

196°C + tested until

failure at RT 842 900 902 32.2 13.7 69

70%) deformed at -

196°C + HT 400°C +

tested until failure at

RT 0 0 1034 0.0 0.0 0

70%) deformed at -

196°C + HT 600°C +

tested until failure at

RT 812 828.5 881 23.6 14.7 68

70%) deformed at -

196°C + HT 800°C +

tested until failure at

RT 308 347.5 621.5 45.5 33.1 71

Tested until failure at

RT 261 294 618 56.8 46 74

70% deformed at RT

+ tested until failure - at RT 749 755 837 30 64

70% deformed at RT

+ HT 400°C + tested - until failure at RT 757 767 823 30 67

70% deformed at RT

+ HT 600°C + tested - until failure at RT 636 663 791 34 63

Tested until failure

at -196°C 390 451 934 89 78 74

70%) deformed at -

196°C + tested until

failure at RT 924 998 1002 29 14 59

70%o deformed at -

196°C + HT 400°C +

tested until failure at

RT 0 0 968 0.0 0.0 0

70%o deformed at -

196°C + HT 600°C + 828 849.5 909 21.8 15.1 52 tested until failure at

RT

70% deformed at - 196°C + HT 800°C +

tested until failure at

RT 315 351 667.5 44.5 34.9 66

Tested until failure at

8 263 314 638 53.8 40

RT 65

70% deformed at RT

+ tested until failure 799 798 817 24 - 65 at RT

70% deformed at RT

+ HT 400°C + tested 838 840 887 23 - 63 until failure at RT

70% deformed at RT

+ HT 600°C + tested 709 733 847 25 - 63 until failure at RT

Tested until failure

360 423 881 67.2 55

at -196°C 67

70% deformed at - 196°C + tested until 849 872 876 42.5 18.4 failure at RT 65

70% deformed at - 196°C + HT 400°C +

tested until failure at

RT 902 902 910 21.4 12.5 55

70% deformed at - 196°C + HT 600°C +

tested until failure at

RT 729 747.5 845.5 26.2 16.8 59

70% deformed at - 196°C + HT 800°C +

tested until failure at

RT 250 288.5 639.5 47.4 35.9 67

Figures 2a to 4b shows the schematic illustration of the tensile properties measured at room temperature for few of the selected alloys of the Example 2. As can be seen from the figures, the yield strength and tensile strength were increased when the alloys were heat treated in the temperature range 400-600°C after cryogenic temperature deformation. Whereas, increasing the temperature beyond the mentioned range significantly decreased the yield and tensile strength (see Table 3). Example 3

In this example, the plastic deformation was achieved by wire drawing at cryogenic temperatures. The effect of heat treatment was investigated for the temperature interval 350 - 800 °C for Alloy 2 of Example 2, The alloy was manufactured by Sandvik.

Surprisingly, as it has been discussed above, Alloy 2 will have stable nanotwins over a larger temperature interval.

Table 4, The chemical composition of the two steel grades in this

investigation.

The wire drawing was carried out at a test bench with a stroke length of 0.8 m. The test bench was equipped with a load cell to measure the drawing force and a position sensor for the drawing speed. On the entrance side where the drawing die was positioned, samples were cooled in a liquid nitrogen reservoir that was directly bolted to the draw bench.

The wire drawing experiments followed the following route:

• Cutting the test material to appropriate lengths

• Cold swaging

• Lubricating the sample

• Mounting the sample in the drawing die

• Cooling down the sample (and where appropriate, extra samples) with liquid nitrogen

• Wire drawing down to an outer diameter of 3.15 mm following the sequence: 5.5 - 4.4 - 3.52 - 3.15 mm, at a speed of 50 mm/s

• Heat treatment

• Vickers indentation measurements on selected samples

Only the wires drawn down to 3.15 mm (ετ = 1.1) was investigated in the heat treatment experiment. The influence of the heat treatment temperature was investigated by heating a 3 cm long wire samples at temperatures from 350 - 800 °C for one hour. The result is displayed in Figure 5.

As can be seen from figure 5, at low heat treatment temperatures the alloy displays an increasing hardness with increasing temperature up to 530 °C. At temperature above 600 °C, there is a decrease of strength (hardness).

Claims

1. A process for manufacturing product of an austenitic alloy comprising the steps of:

a. Providing an object comprising an austenitic alloy comprising the

following composition (in weight%):

C max 0.06;

Si max 2.0;

Mn max 6;

P max 0.04;

S max 0.04;

Cr 15 to 32;

Ni more than 14 to 75;

Mo max 8 ;

W max 8;

N max 0.4;

Cu max 2;

Nb max 0.5;

Ti max 1 ;

balance Fe and impurities b. bringing the object to a temperature below 0°C; c. deforming the object in this temperature; d. heat treating the deformed object in a temperature of from about 400 to about 600 °C during a predetermined period of time.

2. The method according to claim 1, wherein the austenitic alloy is an austenitic stainless steel.

3. The method according to claim 1, wherein the austenitic alloy is an austenitic nickel based alloy.

4. The method according to any one of the preceding claims, wherein the object is brought to a temperature below -50°C before the plastic deformation is imparted to the material.

5. The method according to any one of the preceding claims, wherein the object is brought to a temperature below -75°C before the deformation is imparted to the material.

6. The method according to any one of the preceding claims, wherein the object is brought to a temperature below -100°C before the deformation is imparted to the material, such as below -150°C such as below or equal to -196 °C.

7. The method according to anyone one of the preceding claims, wherein the

deformation is performed by drawing.

8. The method according to any of the preceding claims, wherein the material is plastically deformed to an extent that corresponds to a plastic deformation of at least 50%.

9. The method according to any one of preceding claims, wherein the heat treatment is performed in a temperature of from 400 to 550 °C, such as 400 to 500 °C.

10. The method according to any one of preceding claims, wherein the heat treated object is cooled directly after the heat treatment.

1 1. The method according to any one of preceding claims, wherein the object is heated to room temperature before the heat treatment step.

12. The method according to any one of preceding claims, wherein the product is a tube, a pipe, a strip, a wire, a bar or a plate.