Development of a very high resistance to sensitization in austenitic stainless steel through special heat treatment resulting in grain boundary microstructural modification

Description

Field of Invention

[0001] The present invention relates to a process for achieving enhanced sensitization resistance in austenitic stainless steel by modifying the initial microstructure of the stainless steel through specific heat treatment without involving mechanical treatment.

Background of Invention and Prior Art

[0002] Austenitic stainless steel has a wide application in stainless steel industries (manufacturers, component fabrication), chemical and powder industries. Conventional austenitic stainless steels have a low resistance to sensitization. During fabrication processes such as welding, hot working and stress relieving or during long period of service at elevated temperatures, austenitic stainless steel get sensitized and become prone to intergranular corrosion / intergranular stress corrosion cracking in corrosive environment.

[0003] JP2003253401 discloses a process of thermochemical treatment of austenitic stainless steel to provide excellent intergranular corrosion resistance to austenitic stainless steel without changing chemical components. JP2005015899 discloses a process to provide stainless steel having excellent intergranular corrosion resistance and most suitably used for pipes, structural materials and component parts for use in nuclear power plant. Herein the cited document teaches a chemical composition in order to improve the corrosion resistance and improving the hot workability of the stainless steel. JP2005015896 also teaches a process to provide stainless steel having excellent intergranular corrosion resistance and most suitably used for pipes, structural materials and component parts for use in nuclear power plant. This document teaches a chemical composition in order to improve the corrosion resistance and improving the hot workability of the stainless steel. JP8269550 discloses a chemical composition of an Ni-Cr austenitic stainless steel, in which the content of C is limited to<=0.03% in order to inhibit the precipitation of carbides, causing intergranular stress corrosion cracking, in the grain boundaries and also N having a high degree of ability of entering solid solution is incorporated by <=0.15%, is set. Subsequently at the time of producing this steel, a slab of this steel is heated at a temperature in the range between 1100 and 1300°C, thereby, the amount of precipitation of carbides per unit grain boundary is reduced and also the amount of depletion of Cr in the Cr-depleted region is dispersed. Austenitic stainless steel having excellent intergranular stress corrosion cracking resistance can be provided by the said chemical composition and manufacturing method. The purpose of JP4143214 is to improve intergranular corrosion resistance and intergranular stress corrosion cracking resistance by subjecting an austenitic stainless steel to hot rolling and then allowing Cr carbide to enter into solid solution and recrystallize by means of heating in a specific temperature region. JP1316418 discloses a process to improve the intergranular corrosion resistance of an austenitic stainless steel containing P at a low ratio in a nitric acid solution containing oxidative ions by subjecting said steel to a solutionization heat treatment under specific temperature conditions. The member made from the austenitic stainless steel is subjected to the solutionization treatment by holding the same for >=2 minutes at >=1000°C and rapidly cooling the same at >=40°C/sec cooling rate down to 500°C. The austenitic stainless steel member which has an excellent resistance to the intergranular corrosion even in the nitric acid solution containing ions such as hexavalent Cr ions having high oxidativeness and withstands long-term use as the structural stock in the environment of an apparatus for producing nitric acid, apparatus for treatment of nuclear fuel, etc., where the member is exposed to the nitric acid solution. US 5817193 teaches a product with a grain size not exceeding 30 microns, a special grain boundary fraction not less than 60% and major crystallographic texture intensities all being less than twice that of random values. The product has a greatly enhanced resistance to intergranular degradation and stress corrosion cracking, and possesses highly isotropic bulk properties. Thus there is a need to provide a process of modifying the initial microstructural of the stainless steel to develop a very high resistance to sensitization, which would be of great benefit to stainless steel users.

[0004] The present inventors have surprisingly found that specific heat treatment of the austenitic stainless steel in the temperature range of 1100-1250°C for 30 minutes to two hours without involving mechanical treatment or chemical modifications, results in development of a very high resistance to sensitization in austenitic stainless steel with grain boundary microstructural modification.

Objects of Invention

[0005] It is one object of the present invention to enhance the sensitization resistance of the austenitic stainless steel wrought products during its manufacturing stage by modifying the initial grain boundary nature by specific heat treatment only, without involving mechanical treatment so that a very high resistance to sensitization is achieved in the material.

[0006] Another object of the present invention is that the process does not involve any thermo-mechanical treatment and/or chemical alteration to achieve higher resistance to sensitization.

Summary of Invention

[0007] Accordingly the present invention relates to a process for developing austenitic stainless steel having high resistance to sensitization as measured by the Degree of sensitization, which is reverse of resistance sensitization, and having value of 0.02% to 2.72%, depending upon the types of austenitic stainless steel.

[0008] According to the present invention when sensitization heat treatment was done, degree of sensitization (DOS) which is inverse of sensitization resistance parameter was 1.15%, 0.02%, 2.72% for special heat treated 316LN, 316 and 304 stainless steels respectively as against 11.1%, 0.06%, 8.72% for these steels without special heat treatment, said process comprising exposing wrought stainless steel to specific heat treatment at the temperature range of 1100-1250°C for 30 minutes to two hours and cooling to room temperature.

Detailed description of the Invention

[0009] Conventional austenitic stainless steels have a low resistance to sensitization. The present invention enhances the sensitization resistance of austenitic stainless steel wrought products during its manufacturing stage by modifying the initial grain boundary nature by specific heat treatment in the temperature range of 1100-1250°C without involving mechanical treatment so that a very high resistance to sensitization is achieved in the material.

[0010] By performing a specified heat treatment in the temperature range of 1100-1250°C for duration of 30 minutes to 2 hours without involving mechanical treatment of the austenitic stainless steel, the grain boundary nature is modified to such an extent that chromium carbide precipitation and concomitant chromium depletion are delayed resulting in an increase in the sensitization resistance. Accordingly the present process is less cumbersome as mechanical and chemical alterations are avoided. Further it is economical and has ease of operation.

Working of the invention

[0011] To test the working of the inventive process 3 types of austenitic steel namely AISI type 304 SS (C-0.044%), 316 SS (C-0.054 %) and 316 LN SS (C- 0.03 %, N-0.086%) in the as received mill-annealed condition are subjected to the specific heat treatment. Both as- received as well as special heat treated (according to present invention) specimens of all the three grades are exposed to the so called 'nose treatment' where time required for sensitization is minimum, for time periods > t_min. The degree of sensitization (DOS) is measured by double loop Electrochemical Potentiokinetic Reactivation (EPR) technique.

[0012] DOS is a direct measure of the extent of sensitization. Lower the DOS, higher is the resistance of the material to sensitization. Double loop EPR technique as used to assess Degree of Sensitization (DOS) is defined as below:
DOS = current during reactivation / current during activation X 100

[0013] It is surprisingly found that the sensitization resistance for 304 SS and 316 SS is enhanced by about 3 times and for 316 LN SS is enhanced by about 10 times. Therefore, the specific heat treatment is applicable to the three different varieties of austenitic stainless steels, which are commonly used in industries. The achievement of high resistance to sensitization by specific heat treatment can be generalized for different varieties of austenitic stainless steel.

[0014] The process does not involve any thermo-mechanical treatment on the wrought or fabricated products. The process consists of a specific heat treatment, which can convert a sensitization-prone material to a high resistant material.

[0015] The present invention is further described by way of non-limiting illustrative examples.

Example 1

[0016] 3 types of austenitic stainless steel namely AISI types 304, 316 and 316 LN were subjected to heat treatment in the temperature range varying from <=1000°C, 1000 - 1250°C and >=1250°C for durations of 30 minutes to 2 hrs.

Table1 shows the enhancement in sensitization resistance of 3 varieties of stainless steels after the special heat treatment according to the present invention.

Stainless steel type	Sensitization heat treatment	DOS (%)
		Without special heat treatment (temperature about 1050 ° C)	With special heat treatment (In between 1100-1250 ° C)	Without special heat treatment (1300° C)
304	650°C-1h-Air cooled	8.72	2.72	For all cases there is large grain growth and the resultant material will have a very large grain size. Such materials are not suitable for any engineering applications due to poor mechanical properties and hence not workable
316	750°C-1h-Air Cooled	0.06	0.02
316 LN	650°C-240h-Air cooled	11.10	1.15

[0017] As seen from the example, heat treatment at temperature below 1100 ° C is conventional mill- annealed condition. When such heat treatment is carried out, the data obtained show a low value of sensitization resistance.

[0018] For the heat treatment carried out above 1250° C, there is large grain growth and the resultant material will have a very large grain size. Such materials are not suitable for any engineering applications due to poor mechanical properties and hence not workable as mentioned in the above table. Thus the heat treatment of the present invention is a critical and essential factor, which provides the austenitic stainless steel with highest resistance to sensitization compared to the other treatments outside the range of present invention.

Example 2

[0019] 3 types of austenitic stainless steel namely AISI types 304, 316 and 316 LN are subjected to heat treatment in a temperature range of 1100 to 1250° C for a varying duration of time.

Stainless steel type	Sensitization heat treatment	DOS (%)
		Duration (15 minutes)	Duration (30 minutes to 2 h)	Duration (3h)
304	650°C-1h-Air cooled	4.52	2.72	4.15
316	750°C-1h-Air Cooled	1.38	0.02	2.00
316 LN	650°C-240h-Air cooled	3.91	1.15	2.59

[0020] As seen from the above example, heat treatment for a duration of 15 minutes and 3 hours results in higher degree of sensitization (%) and hence not workable. Thus the duration of heat treatment of the present invention is a critical and essential factor which provides the austenitic stainless steel with highest resistance to sensitization compared to the other treatments outside the range of present invention.

Claims

1. A process for developing austenitic stainless steel having high resistance to sensitization as measured by degree of sensitization (DOS) which is reverse of resistance sensitization and having value of 0.02 to 2.72 %, depending upon the types of austenitic stainless steel, wherein said process comprises exposing wrought stainless steel to specific heat treatment at the temperature range of 1100-1250°C for 30 minutes to two hours and cooling to room temperature.

2. A process as claimed in claim 1 wherein degree of sensitization for 316LN steel formed in the process is 1.15%.

3. A process as claimed in claim 1 wherein degree of sensitization for 316 steel formed in the process is 0.02%.

4. A process as claimed in claim 1 wherein degree of sensitization for 304 steel formed in the process is 2.72%.

5. A process as claimed in claim 1 wherein said exposure to heat treatment is without thermo mechanical treatment or chemical modifications.

6. A process as claimed in the preceding claims wherein grain boundary microstructural modification occurs leading to increase in resistance to sensitization of the austenitic stainless steel.

7. The process as claimed in claim 3 wherein the said grain boundary nature is modified such that chromium carbide precipitation and concomitant chromium depletion are delayed.