Solid solution nitriding (also known as high temperature gas nitriding) is a versatile, simple, promising high-nitrogen stainless steel solid-state production process. However, in the solid solution nitriding process, the material is exposed to high temperature for a long time, and coarse austenite grains are formed in the microstructure, which deteriorates the mechanical properties of the material.
This paper mainly studies the grain refinement process of nickel-free manganese Fe-22.75Cr-2.42Mo-l.17N austenitic stainless steel plate. Austenite grains were refined by studying two-stage (decomposition-re-austenification) isothermal heat treatment. Uniform preheat treatment was used in the experiment, and the effect of heat treatment on the grain refinement process of steel was studied.
1 experimental process
A 2 mm thick nickel-free manganese Fe-22.75Cr-2.42Mo-1.17N austenitic stainless steel sheet was produced by solid solution nitriding at 1200 ° C under a nitrogen pressure of 0.25 MPa. The Fe-Cr-Mo alloy was prepared by induction heating melting, the atmosphere was argon gas, followed by electroslag remelting, and homogenization treatment was carried out at 1100 ° C for 48 h. Table 1 shows the chemical composition of the Fe-Cr-Mo steel in this study. A 10 mm thick slab was hot rolled into an initial ferritic steel sheet at 1000 °C. Before solid solution nitriding, the steel plate needs to be annealed at 900 ° C for 1 h.
At the time of the test, N was added to the ferritic stainless steel plate by solid solution nitriding in a horizontal tube furnace. The furnace was first purged with nitrogen for 15 minutes, and then heated at a rate of 10 ° C / min to continuously introduce nitrogen into the furnace. When the temperature reaches 1200 °C, the nitrogen pressure in the furnace rises to 0.25 MPa (2.5 bar), the pressure is maintained, and the sample is subjected to solid solution nitriding for 1, 3, 6, 9, 12, 13 and 18 h, respectively. The nitrogen sample was water quenched.
Samples of solid solution nitriding, homogenization and grain refining steel are denoted as SN-12H, SN-12H-H and SN-12H-R, respectively.
2 results and discussion
2.1 Ferrite-austenite transformation through solid solution carburizing
The typical optical microstructure of the ferritic steel plate before and after solid solution nitriding shows that the cross-sectional microstructure of the substrate consists of equiaxed ferrite grains with an average grain size of about 130 μm before solution nitriding. When the sample is subjected to solid solution nitriding for a long time, the ferrite in the structure is completely transformed into austenite, the phase transformation is throughout the thickness of the steel plate, and there are coarse austenite grains (hundreds of micrometers) and certain in the structure. The amount of twins (caused by nitrogen absorption during solid solution nitriding), no chromium nitride phase (Cr2N and CrN) was detected by XRD analysis.
The intensity-time curve analysis of the initial ferrite sample and the solid nitrogen content of the sample after solid solution nitriding (austenite) gives an initial nitrogen content of 0.02 for the initial ferritic and austenitic steels, respectively. % and 1.17% (mass percentage). This indicates that the initial ferritic steel has absorbed more than 1% of nitrogen, which is sufficient to promote the transformation of the ferrite phase into the austenite phase.
2.2 Refinement of austenite grains by isothermal heat treatment
The sample after solid solution nitriding is not uniform in microstructure and coarse grains. This organization can deteriorate the mechanical properties of HNASS. In order to obtain a more uniform microstructure and improve mechanical properties, a two-stage isothermal heat treatment process is used to refine austenite grains. According to the calculated Fe-22.75Cr-2.42Mo-(2-3)N phase diagram, the ferrite (a) and chromium nitride (Cr2N) phases are stable in the temperature range of 25-1000 °C. Above 1080 ° C, austenite is the equilibrium phase.
From the phase diagram of Fe-22.7Cr-2.42Mo-(0-2)N alloy calculated by Thermocalc software and TCFE6 database, austenite fine crystals can be formed by two-stage heat treatment process: (1) The austenitic steel with coarse grains is isothermally heated to below 1000 ° C, the austenite is decomposed into ferrite (a) and chromium (Cr2N) phases, and (2) is subsequently in the temperature range of 1080 to 1250 ° C. The decomposed steel is re-auped and then water quenched.
In order to suppress the formation of the γ phase and the χ phase, 740 to 1000 ° C seems to be a suitable heat treatment temperature window for austenite isothermal decomposition by γ → α + Cr 2 N eutectoid transformation. Due to the lack of empirical TTT map, and it is difficult to theoretically calculate the graph, the SN-12H steel samples were subjected to 5 min, 10 min, 15 min, 1 h, 2 h, and 4 h at 550 ° C, 700 ° C, and 900 ° C. Warm heat treatment. The process of nitrogen eutectoid transformation from the metallographic structure of the sample cross section revealed that the TTT curve nose was located at about 900 ° C, where austenite was completely decomposed in a short time.
2.3 Decomposition of solid solution nitriding (austenitic) steel
The microstructure of the SN-12H steel heated at 700 ° C for 4 h showed that the decomposed tissue was white or black. High-magnification SEM images of these regions indicate that austenite in these two regions decomposes into ferrite and chromium nitride precipitates. However, the white region precipitates less chromium nitride than the black region. The energy spectrum analysis of these regions shows that the nitrogen in the black region is higher than the white region.
2.4 Re-austenification of decomposed steel
The microstructure of the decomposed steel plate was re-austenification + water quenching at 1200 ° C for 0.5 h, and the microstructure of the sample corner and core was analyzed. It was found that ferrite to austenite occurred in the thickness of the whole sample. The transformation. The impact between a large number of grown austenite crystals leads to the formation of fine grained structures. However, there is a two-phase zone (ferrite + austenite) in the core of the sample, which indicates that the ferrite is not completely transformed into austenite.
In addition, the high-magnification microstructure of the corners of the sample indicates that there are spherical or flaky particles in the newly formed austenite, and some of these particles are precipitated by chromium nitride. Therefore, it seems that two processes occur simultaneously: the transformation of ferrite into austenite, and the dissolution of chromium nitride in the austenite lattice.
When the re-austenization time is increased to 1 h, all ferrite is transformed into austenite, and chromium nitride is dissolved in the newly formed austenite and becomes smaller and smaller. However, there are still chromium nitride particles in the austenite grains at the corners of the sample. This means that the transformation of ferrite to austenite has ended before the chromium nitride precipitate is completely dissolved. Therefore, at 1200 ° C, it takes a longer holding time for the chromium nitride precipitate to dissolve sufficiently. In this case, the newly formed austenite grains are coarsened due to re-austenization due to exposure to a high temperature environment.
2.5 Decomposition of homogenized austenitic steel
Solid steel increases nitrogen by absorbing nitrogen to cause an uneven distribution of nitrogen in the steel. The root causes of this non-uniformity are: (1) the nitrogen content of the surface and corners of the sample is the highest, and the nitrogen content gradually decreases toward the core of the sample. (2) During the nitriding process, the nitrogen diffusion and nitrogen concentration depend on the grain orientation. (First the initial ferrite grain orientation, followed by the transformation of the austenite grain orientation).
In order to obtain a grain-reduced, non-precipitating microstructure, the complete austenitizing (SN-12H) sample should have a uniform nitrogen concentration across the cross section. This homogenization process requires special conditions. Therefore, the SN-12H samples in this study were heat treated at 1200 ° C for 10 h under a nitrogen pressure of 0.1 MPa to prevent nitrogen from occurring in the steel before grain refinement treatment. Loss and precipitation of chromium nitride. After homogenization heat treatment, the microstructure of a typical SN-12H sample (SN-12H-H) is equiaxed austenite grains with an average grain size of 100 μm.
After the SN-12H-H sample was subjected to isothermal heat treatment at 700 ° C for 1 h and 4 h, chromium nitride was mainly precipitated by grain boundary and nucleation around the inclusions, and was in the form of aggregated structure. The eutectoid structure is first formed, and then the aggregated tissue is grown into the austenite grains. After isothermal heat treatment at 700 °C for 4 h, the SN-12H-H sample was completely transformed into eutectoid structure, and the precipitate of Cr2N in the eutectoid structure showed a band-like morphology. This eutectoid structure is similar to the pearlite structure in carbon steel and can be called nitrogen-containing pearlite. On the other hand, the decomposition structure of the SN-12H-H sample was more uniform than that of SN-12H.
2.6 Microstructure of grain refining steel
The optical microstructure of the grain refining steel showed that the SN-12H-H sample containing the (α+Cr2N) decomposed structure was re-austenitized at 1200 ° C for 10 min, followed by water quenching to obtain the average grain size. Fine-grained austenite having a size of 20 μm. The effect of intermediate homogenization on reducing the re-austenification time.
2.7 Microhardness of grain refined steel
Vickers microhardness test of substrate (nitrogen-free ferritic steel), solid solution nitriding steel after homogenization heat treatment (SN-12H-H sample) and grain refinement (SN-12H-R sample) The results show that the hardness of SN-12H-H sample is much higher than that of nitrogen-free steel and traditional AISI316L steel. When the grain size is reduced from 100μm to 20μm, the microhardness increases from 270HV0.5 to 347hV0.5.
2.8 Austenite decomposition of nickel-free manganese austenitic steel and nickel-free austenitic steel
Compared with traditional biomedical nickel-free HNASSs (>12%), the nickel-free manganese HNASS in this study has different Cr2N precipitation behavior during austenite decomposition for the following reasons:
1) During the isothermal heat treatment, the supersaturated austenite of the conventional nickel-free HNASSs becomes unstable and decomposes into denitrified austenite and Cr2N. However, in the isothermal heat treatment of nickel-free manganese HNASS below 1000 ° C, austenite is decomposed into ferrite and Cr 2 N.
2) In nickel-free HNASSs, Cr2N nucleation and growth occur through discontinuous or honeycomb precipitation (ie, γ supersaturation → γ saturation + Cr2N), however, nickel-free manganese HNASS needs to undergo eutectoid transformation (ie: γ → α + Cr 2 N) diffusion of chromium and nitrogen to achieve precipitation of Cr 2 N.
3) In the isothermal process, such as aging, hot forging and welding, the precipitation of Cr2N in nickel-free HNASSs will seriously damage the mechanical properties and corrosion resistance of steel. However, the γ→α+Cr2N eutectoid transformation occurs without nickel-manganese HNASS. , can effectively refine the grain.
3 conclusions
In this paper, the grain refinement of nickel-free manganese Fe-22.75Cr-2.42Mo-1.17N high nitrogen austenitic stainless steel sheet produced by solid pressurized solid solution nitriding method was studied by two-stage heat treatment process, and the following conclusions were drawn. :
(1) During the isothermal heat treatment of <1000 °C, Fe-22.75Cr-2.42Mo-1.17N alloy undergoes γ→α+Cr2N eutectoid transformation, and most of the chromium nitride dispersed in the ferrite matrix Nucleation at the grain boundary forms an eutectoid aggregate structure. The precipitates of Cr2N in the nitrogen-containing eutectoid structure have a band-like morphology.
(2) The nitrogen-containing pearlite of the solid solution nitrided steel has uneven distribution of Cr2N precipitates. This may be related to the uneven distribution of nitrogen.
(3) When austenitizing at 1200 ° C for 1 h, the Cr 2 N nitride was not completely dissolved, which was caused by uneven distribution of nitrogen in the solid solution nitrided steel sheet.
(4) After homogenization heat treatment at 1200 ° C for 10 h, a uniform eutectoid structure can be obtained.
(5) By subjecting the entire section to nitrogen homogenization treatment and grain refining heat treatment, the grain size of the nickel-free manganese austenite can be successfully refined to 20 μm, and the microhardness value reaches 347 HV0.5.
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