Control of surface hardnesses, hardening depths, and residual stresses of low carbon 12Cr steel by flame hardening

The properties of hardness, hardening depth and residual stress of low carbon 12Cr steel have been investigated after the flame hardening modification. For this, the temperature cycles of 12Cr steel have been controlled precisely as a function of the surface temperature, the exposed height from the water surface, and the cooling rate. Flame hardening process increases the hardness of 12Cr steel (base value, approx. 250 HV0.2) up to 420-550 HV0.2 considerably, forming relatively sharp hardening boundaries. Both the hardening depth and the interaction time are linearly correlated well within the range of the processing conditions employed. The rapid cooling rate also increases the hardening depth as well as the surface hardness. The properties of residual stress generated by flame hardening are discussed in terms of contributions of both the thermal and transformation stress components. The optimum processing conditions for the desirable residual stress state in the material were <1200 degreesC for the 18 mm-exposed and <1150 degreesC for the full-exposed conditions by the criteria of Siemens AG-KWU, and on the basis of the specification of GE Power Generation Engineering, they were 944-1050 degreesC for the 10 mm-exposed, 883-1150 degreesC for the 14 mm-exposed, 833-1134 degreesC for the 18 mm-exposed, and 785-1097 degreesC for the full-exposed. (C) 2003 Elsevier B.V. All rights reserved.