Influence of nickel on mechanical properties of in structural alloy steel.

In order to supply oil and gas producing industry with oil pumps that are frequently operated in the Far North at significant low temperatures, the manufacturer of components of forged raw parts (shafts, gears, couplings etc.) has a task to provide raw parts of structural steel grades with a set of mechanical properties that allow for operation under minus temperatures. The forging facility that is a part of KONAR Industrial Group faces this problem. When producing shafts for motors of the oil pumps, the achieved values of impact energy at low temperatures were chaotic and, as a rule, did not meet the requirements of the material specifications. However, strength characteristics of the material conformed to the standard. The article studies influence of nickel on mechanical properties in the structural steel 40ХН2МА. The results of mechanical testing of two test heats with nickel content at the low and top levels of reference documentation are given. Within the research work, two shafts were produced of forging ingots of both heats of steel 40ХН2МА by open-die forging method. The modes of metal forming and heat treatment for achieving the required properties were identical. The sampling was performed according to GOST 8479–70 on each test forging. Comparison of the results of impact testing of test heats metal at low temperatures shows positive impact of high nickel content on impact strength. With the difference in nickel content of 0.29% the average value of impact strength increased for 66.6 J/mm2. The average value of ultimate strength increased for 80 MPa, yield strength for 90 MPa. The microstructure analysis demonstrated the difference in the ferrite phase, i.e. when the nickel content increases, the ferrite phase fraction increases, and the degree of secondary sorbate dispersion increases.