Research on Vertical Extrusion Process of TP321 Austenitic Stainless Steel Seamless Pipe
By dealing with thermal simulative tester GLEEBLE3500 ，TP321 austenitic stainless steel has been con ducted isothermal and constant strain compression test and effect of deformation degree and extrusion temperature on ac tual grain size have been analyzed． Reasonable technical parameter has been verified and identified in trial production ， which provided technical basis and support for vertical extrusion process establishment of TP321 austenitic stainless steel seamless pipe．
Key words: TP321 austenitic stainless steel ; extrusion process ; grain size; deformation
In the past, China's demand for TP321 austenitic stainless steel large-diameter thick-walled seamless steel pipes mainly relied on imports. In recent years, with the development of science and technology level and manufacturing level, relevant domestic enterprises have begun to conduct research on the localized production process of TP321 austenitic stainless steel seamless steel pipe, and have initially mastered the production process of TP321 austenitic stainless steel seamless pipe. Replace imported products with domestic products. When extruding TP321 austenitic stainless steel large-diameter thick-walled seamless steel pipe with a 360 MN vertical extrusion machine, in order to make the grain size meet the technical requirements, it is necessary to carry out targeted adjustments on the characteristics of this type of material and its extrusion process parameters handicraft Research. In this paper, the relationship between extrusion force and deformation degree of TP321 material and extrusion temperature has been studied, and a reasonable vertical extrusion process for seamless steel pipes of this type of material has been formulated.
- Test materials and methods
In order to understand the deformation resistance of TP321 austenitic stainless steel in the hot state and to determine the feasibility of vertical extrusion, an isothermal and constant strain compression test was carried out using the American thermal simulation test machine ( GLEEBLE3500-10/1-75 ) to understand the The stress of a material in its thermally deformed state. Cut the austenitic stainless steel test piece and process it into a cylindrical sample of $ 10 mm X 12 mm . During the thermal compression test, graphite sheets are placed on both ends of the sample to reduce the friction of the end surface and avoid obvious uneven deformation such as waist drum and rollover. When the sample is compressed, the deformation temperatures are 950°C , 1 050°C , 1 070°C ,
1 100C , 1 200C and 1 250C , the strain rates are 0.1 s - 1 , 1 . 0 s - 1 , the degree of deformation (high compression rate) are 35% , 40% , 45% , 50% , 55% , 60% respectively . The heating rate of the sample is 10C/s, the heating and holding time is 300 S, and then compressed. Both heating and deformation are carried out under vacuum conditions, and air cooling is adopted after compression deformation. During the deformation process, the temperature and strain rate are automatically controlled by the computer of the thermal simulation testing machine to keep constant. The load - travel curve obtained directly from the test is collected and trimmed by special software, and the stress-strain data is directly output.
- Test Results and Analysis
- 1 Analysis of thermal simulation test results
It can be seen that when the strain rate is constant, the flow stress decreases as the extrusion temperature increases, which indicates that the flow stress of TP321 austenitic stainless steel is more sensitive to the deformation temperature. However, as the degree of deformation increases, the flow stress also increases. At 950C , even if the degree of deformation is only 35% , its flow stress is still as high as 280 MPa ; at 1 050 ~ 1 100C , the flow stress of 60% deformation is ( 210 ~ 230 ) MPa ; at 1 200 ~
1 250° C , the flow stress of 60% deformation is about ( 75 - 150 ) MPa . Therefore, choosing a smaller degree of deformation and a higher temperature can reduce the flow stress during the deformation process. Under the condition of ensuring the intrinsic quality of the product, it is particularly important to select an appropriate heating temperature and degree of deformation.
From the analysis of the -strain curve, it can be concluded that under the condition of meeting the technical requirements of the extruded product, using a relatively small amount of deformation can avoid excessive extrusion force during the extrusion process. As long as the actual grain size of the product meets the technical requirements, it is feasible to choose extrusion at 1 050 - 1 200C .
2.2 Analysis of metallographic results
TP321 austenitic stainless steel pipe products are required to inspect the grain size according to the ASTM E112 standard, which is stipulated as grade 5~7 . The actual grain size of steel pipe products produced by extrusion is closely related to the extrusion temperature, and the extrusion temperature determines the actual grain size. Choosing a higher extrusion temperature can reduce the extrusion force and relieve the pressure of the extruder, but it will also coarsen the actual grain size, which cannot meet the technical requirements of steel pipe products. To this end , it is very important to choose a reasonable extrusion temperature between the allowable 1 050 ~ 1 200C , which is very important for the actual grain size of the extruded tube blank and the protection of the extrusion equipment . 1 The detection of grain size at 200C thermal simulation extrusion temperature selects the extrusion temperature that can make the actual grain size of steel pipe products meet the technical requirements.
Through the thermal simulation test machine test and combined with the metallographic analysis results, it can be clearly seen that under the condition of 950C , the uneven deformation leads to serious mixed crystal phenomenon, especially the grains in the white band area are coarse, reaching grade 1 , the finest 0~6. 0级。 It is also only 4. 0 ~ 6. 0 level. This shows that low extrusion temperature leads to insufficient deformation.
In summary, in order to meet the technical requirements of steel pipe products,
TP321 austenitic stainless steel is determined to be 1 070-1 200C , which not only meets the requirements of actual grain size, but also has small deformation resistance.
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