Methods to prevent quenching cracks in EN10216-2 alloy steel pipes

　　(A) If the entire EN10216-2 alloy steel pipe is quenched by strong water If the steel pipe is cooled to the martensitic transformation termination temperature (Mf point) or earlier, quenching cracks are likely to occur.

　　(B) Since the crack during quenching cracks roughly extends along the gap of the EN10216-2 alloy steel pipe, it can be considered that the main force for crack extension is the tensile stress in the circumferential direction.

　　(C) Regarding the source of the above-mentioned circumferential direction tensile stress, it is considered that the temperature difference (temperature unevenness) in the wall thickness direction generated during the cooling process causes spurs on the outer surface side and the inner surface side of the steel pipe. There is a bias in the timing of the transition.

　　(D) Especially near the cooling surface where the temperature unevenness is large (that is, the temperature difference with the inner surface side is large), microcracks due to brittle fracture are likely to occur, and the microcracks are likely to become the starting point and crack propagation point of the crack.

　　(E) In most cases, cracks occur in EN10216-2 alloy The end of the steel pipe is the starting point for extension. The reason is considered to be that the stress increase coefficient at the end portion having the free surface is larger than the stress increase coefficient at other than the end portion.

　　(F) When the cooling rate is suppressed without water cooling, quenching cracks do not occur in low alloy steel and Cr-based stainless steel containing high carbon. For EN10216-2 low-alloy steel pipes containing high carbon, if martensite sensitization is suppressed and the structure is changed to a Bainite structure, quenching cracks will not occur. In short, it can be considered that in most cases, quenching cracks start from cracks generated at the end of the steel pipe with a free surface.

　　(G) Even for low-alloy steel or medium-alloy steel pipes that are prone to quenching cracks during water quenching, sufficient Martens hardness in the steel pipe can be ensured if the end of the steel pipe is not water-cooled. Except for the ending. If the cooling rate of the volume ratio is water cooling, water quenching can be performed stably without quenching cracks.

　　(H) When the above water quenching method is applied to steel pipes made of martensitic EN10216-2 alloy stainless steel pipe, high performance can be ensured without quenching cracks. A steel pipe quenching method is characterized in that it is a steel pipe quenching method that performs water-cooling quenching from the outer surface, in which the pipe end is not water-cooled, but at least part of other parts are water-cooled.