Effects of overheating environment on austenitic stainless steel tubes.

In the heat treatment production, must all martensitic hardening 301 stainless steel sheettube, and it must be the state of austenitic steel tube to equal or exceed the cooling speed of V for cooling, the austenitic stainless steel pipe even in nasal would not have time decomposition temperature, cold until Ms points, austenite into martensite. The minimum cooling speed of the supercooled austenitic stainless steel tube is usually called the critical cooling rate. The Vk given by the C curve of different steel types is the main basis for determining the quenching method and choosing quenching medium, but how to obtain Vk from the C curve? If the cooling rate of continuous cooling when painting on the isothermal cooling curve C, in order to determine vitamin k is only approximate approach, because of isothermal cooling curve C and continuous cooling supercooled austenite stainless steel tube shift there is a difference between (continuous cooling should be C curve moves to the right). In addition, the assumption is needed here, when the quenching cooling, the temperature drop into a direct ratio with time, then there is the following relations: but this always calculated V 1.5 times greater than the actual determination of vitamin k about, and joined in the type correction coefficient of 1.5, was the critical cooling rate.

Determination of usually continuous cooling transformation diagram than isothermal curve C is much more difficult in technology, therefore, can make use of isothermal C curve approximation to speculate that under the condition of continuous cooling transformation process of the austenitic stainless steel pipe and its products. Its method is the effective diameter or thickness of stainless steel pipe in different cooling hardening agent, to measure the cooling curve of fold draw on isothermal curve C, see its intersection with isothermal curve C, can be qualitatively determined with certain diameter of the steel in different transformation temperature of quenching medium and its product range, so as to guide our make heat treatment of the cooling process, for example, figure 10 to 18 and figure 10-19 respectively crnimov cold deformation of die steel stainless steel tube and 75 C curve, and respectively on the fold draw west 20 mm sample in the oil and air cooling of parts surface and center of the cooling curve, So by figure 10 to 18, the wall thickness of 20 mm 302 stainless steel sheet tube, if is cooled in the air will be within the range 650-600? into pearl body organization, only about HRc26 hardness. If in oil quenching, the parts surface is about 590? or so part into pearlite organization, through metallographic analysis its volume accounts for about 15%, while the rest of the supercooled austenite stainless steel pipe are all transformed into martensite under 240?, thus the surface to obtain martensitic (85%) and pearl shape (15%) of hybrid, hardness is about HRc59. And the heart of all within the range 660-530? into pearl body organization, only HRc37 hardness. This indicates that the cooling speed of air and oil has not reached the critical cooling rate of the stainless steel tube with a diameter of 420 mm. Since carbon steel is a kind of water hardening steel, any medium with less cooling speed than water will inevitably get a certain proportion of pearlescent body tissue in its tissues, so it cannot be fully quenched.

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