The Influence Of Overheating Environment On Austenitic Stainless Steel Pipe

In the heat treatment production, in order to quench the stainless steel tube to obtain all martensite, the austenitic steel tube must be cooled at a cooling rate equal to or higher than V, so that the austenitic stainless steel tube is also at the nasal temperature. Before it can be decomposed, the austenite is transformed into martensite until it is cooled below the Ms point. Usually the minimum cooling rate Vk of the supercooled austenitic stainless steel pipe is called the critical cooling rate.

In this way, the Vk given by the C curve of different steel grades is the main basis for us to determine the quenching method and select the quenching medium, but how to obtain Vk from the C curve? If the cooling rate during continuous cooling is superimposed on the C curve of isothermal cooling, it is only an approximation to determine Vk, because the C curve of isothermal cooling is different from the transformation of supercooled austenitic stainless steel pipe during continuous cooling. Different (the c curve should be shifted to the right during continuous cooling). In addition, it needs to be assumed here that during quenching and cooling, the temperature drop is proportional to the time, then there is the following relationship: but the calculated V is always about 1.5 times larger than the actual measured Vk, so a correction is added to the formula With a coefficient of 1.5, the critical cooling rate is obtained.

It is usually more technically difficult to determine the continuous cooling transformation diagram than the isothermal C curve. Therefore, the isothermal C curve can be used to approximate the transformation process and products of austenitic stainless steel pipes under continuous cooling conditions.

The method is to cool a stainless steel tube with a certain effective diameter (or thickness) in different quenching agents, overlay the measured cooling curve on the isothermal C curve, and look at the intersection point with the isothermal C curve to determine qualitatively The transformation temperature range and products of this steel with a certain diameter in different quenching agents guide us to formulate the cooling process of heat treatment, which is the C curve of the stainless steel tube and the 75CrNiMoV cold-deformed die steel, and superimposes the west on it. The cooling curve of the surface and the core of the part when the 20mm sample is cooled in oil and air. As shown in Figure 10-18, the stainless steel pipe with a wall thickness of 20mm will be in the range of 650-600℃ if it is cooled in air. All the inner parts are transformed into pearlite tissues, and the hardness is only about HRc26.

If it is quenched in oil, the surface of the part will be partially transformed into pearlite structure at about 590℃, and its amount will account for about 15% through metallographic analysis, while the rest of the supercooled austenitic stainless steel tube will be transformed into pearlite structure below 240℃. Martensite, so as to obtain a mixed structure of martensite (85%) and pearlite (15%) on the surface, with a hardness of about HRc59. The heart is completely transformed into pearlite-type tissue within the range of 660-530℃, with a hardness of only HRc37. This shows that the cooling rate of air and oil did not make the 420 mm outer diameter stainless steel tube reach the critical cooling rate of quenching. Since carbon steel is a water-hardened steel, any medium with a cooling rate less than water will inevitably obtain a certain percentage of pearlite structure in its structure, and therefore cannot be completely quenched.

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By Minghe Die Casting Manufacturer |Categories: Helpful Articles |Material Tags: Aluminum Casting, Zinc Casting, Magnesium Casting, Titanium Casting, Stainless Steel Casting, Brass Casting,Bronze Casting,Casting Video,Company History,Aluminum Die Casting |Comments Off