High chromium martensitic stainless steels (MSS), for example 13Cr steel, are widely used as OCTG tubing material for corrosive oil and gas wells. These materials are manufactured by quenching and tempering processes. Recently, the opportunities when the materials experience additional heat treatment (reheating) increase. However, influence of reheating on mechanical properties has not been clarified yet. In this study, influences of reheating on mechanical properties of martensitic stainless steel were investigated in order to clarify the behavior of 475 °C embrittlement in these materials. In addition, appropriate heat treatment conditions for these materials are proposed to provide martensitic stainless steel with good mechanical properties after reheating.
Three kinds of martensitic stainless steel pipes (0.2C-13Cr, 13Cr-5Ni-0.6Mo, and 12Cr-5.5Ni-2Mo) manufactured by quenching and tempering processes were used in this study. Several additional heat treatments were applied to these materials for simulating reheating by changing the temperature from 315 to 482 °C (600 to 900 °F) for 24 hours. Tensile test, hardness test and Charpy impact test were carried out to evaluate the change of mechanical properties by reheating. Furthermore, laser-assisted wide angle three-dimensional atom probe (LA-3DAP) was employed in order to observe the chromium-rich cluster produced by long-term reheating at a specific temperature condition.
The effect of reheating on mechanical properties of 0.2C-13Cr steel was not observed clearly in the range of test conditions. Modified 13Cr martensitic stainless steel containing nickel showed the increase of YS, TS, and hardness. In addition, deterioration of toughness by reheating around 400 °C for 24 hours was observed. The LA-3DAP analysis on modified 13Cr steel showed that chromium-rich clusters of a few nanometer size existed. To explain the 475 °C embrittlement of martensitic stainless steel, a thermodynamic calculation of critical condition of bcc phase separation was also conducted. From the results of the calculation and the estimated effective chromium of steel, the effect of reheating temperature on the embrittlement for martensitic stainless steel was discussed.
For martensitic stainless steel, reheating around 400 °C should be avoided because of 475 °C embrittlement. When the additional heat treatment is inevitable, the process condition should be carefully reflected to minimize the embrittlement, for example, low temperature, high temperature, and/or short duration.