Selection of reference specimens for assessing the efficiency of part surface nitriding

The use of corset reference specimens for carrying out fatigue tests and smooth reference specimens for determining residual stresses in a surface layer to assess the sufficiency of nitriding (thermochemical processing) is substantiated in the paper. The calculations are carried out by finite element simulation using the ANSYS and PATRAN/NASTRAN software complexes. The ANSYS complex is used to determine the stress concentration factor for the case of corset specimen bending. The employed models with harmonic axisymmetric finite elements make it possible to find the stress concentration factors in the case of tension- and- compression and bending of body-of-rotation parts. The obtained value of the influence of the coefficient of residual stresses on the endurance limit increase is in quite good agreement with its experimental values for nitriding and other types of thermochemical treatment of corset specimens. The PATRAN/NASTRAN complex is used to calculate the stress-strain state in corset specimens depending on residual stresses arising on a part surface under nitriding. Modeling is performed by the thermoelasticity method, with experimental diagrams of residual stresses in smooth specimens used as initial data. It is established that the distribution of axial residual stresses in the dangerous (smallest) section of a corset specimen practically does not differ from the distribution in smooth specimens. The results obtained provide the basis for determining average integral residual stresses by the distribution of residual stresses in smooth specimens and using them to calculate the endurance limit for hardened corset specimens. The results of the study allow making a conclusion that the most optimal nitriding practices are those wherein the average integral residual stresses are maximal.