The medium and high temperature equipment in oil refini […]
The medium and high temperature equipment in oil refining, large thermal power units and pressurized gasification units for syngas production from coal to oil all require steel to have good oxidation resistance (heat resistance), sufficient high temperature strength and excellent toughness. For this reason , Various types of chromium molybdenum steel are widely used.
Through some experimental studies and engineering examples, the impact of pressure-bearing equipment made of chromium-molybdenum steel on the properties of steel after overall heat treatment is introduced. For this reason, the overall post-weld heat treatment process of chromium-molybdenum steel equipment was discussed and some suggestions were put forward.
At present, some large and medium-sized manufacturing plants in my country have more steel grades for manufacturing such equipment: 1Cr05Mo (for example: 15CrMoR, 15CrMog, SA387Grl2, etc.); of course, there are also Cr-Mo steels containing vanadium. In addition, some oil refining equipment is made of explosive clad steel plate, the base layer is Cr-Mo steel, and the clad layer is stainless steel.
Cr-Mo steel equipment requires overall post-weld heat treatment to eliminate welding residual stress, reduce the hardness of the welded joint and improve its mechanical properties. With the increase in the strength or thickness of the large-scale steel of the device, the manufacturing process specifications may increase the heat treatment temperature or extend the holding time.
The Cr-Mo steel weld metal contains more hardened structure. After a higher temperature overall heat treatment, the toughness is improved while eliminating the stress and reducing the hardness. However, increasing the overall heat treatment temperature or excessively prolonging the holding time (that is, increasing the tempering parameter) will cause carbides in the metallographic structure of the weld metal and the heat-affected zone to accumulate along the grain boundaries, and may also cause the ferrite grains to be coarse change. The strength of the welded joint is reduced, the toughness becomes worse, and stress embrittlement (reheat embrittlement) occurs.
The brittle transition temperature of the weld metal is the lowest (the highest toughness); the brittle transition temperature of the three weld metals has been improved to different degrees (the toughness becomes worse, that is, the reheat embrittlement). The reheat embrittlement tendency of 25Cr-1Mo is very slight. When it is subjected to higher temperature and longer heat treatment, the curve rises less obviously, and the toughness remains at a higher level. This is in line with the Cr, The high content of Mo is consistent with the performance of strong heat resistance; 25Cr-0.5Mo has an obvious tendency to reheat embrittlement. When the T value starts from about 19.5, the curve rises significantly, which is not high toughness. There has been a sharp drop. However, since the vTrs value of the weld metal of the 1.25Cr-0.5Mo steel is almost the same before and after the step-cooling test, it shows that its temper brittleness is very small. For this type of steel, the content of C, Si, and Mn is generally increased or adjusted. Improve hardenability and reduce oxygen content to increase strength and toughness, instead of improving tempering parameters to achieve the goal.