Investigations of Residual Stress Distributions in Retained Austenite and Martensite after Carbonitriding of a Low Alloy Steel

Abstract

Abstract. Specimens of low alloy steel were carbonitrided under different conditions to attain varying levels of carbon and nitrogen contents. The residual stress depth distribution was evaluated in martensite and retained austenite by X-ray diffraction. Beside standard evaluations, triaxial residual stress states with σ 33 ≠0 in both phases were also considered. High values of residual stresses in both phases were observed. The sign, magnitude and location of maximum compressive residual stresses were greatly influenced by the level of carbon and nitrogen contents. IntroductionThe carbonitriding process is the modified form of carburizing [1] during which ammonia gas is added into the carburizing atmosphere. The main advantage of this process in comparison to carburizing is the rapid diffusion of carbon (C) and nitrogen (N) as well as the formation of high compressive residual stresses (RS) in the case. High compressive RS and hardness enhance the fatigue properties of components like shaft, gear and bearing and improves resistance to wear, bending fatigue, and rolling fatigue [2]. In recent years, several studies [3,4] have focused on the investigation of carbonitriding process and the resulting residual stresses (RS) in the case. Even though to date, still nearly all published data on RS belong to martensite/bainite phase and only little information on retained austenite (RA) residual stress state in carbonitrided specimens is available. Nowadays, however, the trend is toward retaining high percentage of austenite in the case aiming at improved contact, bending and impact fatigue. It is evident that due to high amount of RA (>30 Vol. %), the sign, magnitude and distribution of RS in both martensite and RA phase will be influenced and such influences may affect the mentioned properties [5]. Therefore, the RS in martensite and in austenite have to be considered. Varying the level of C+N content in solution affects not only the amount of RA but also the magnitude and distribution of RS. Therefore, a need arises for thorough understanding the effect of different level of C and N contents on the sign, magnitude and distribution of RS. Moreover, it is still unclear whether the residual stress state measured by laboratory XRD in a multiphase material is a plane stress state due to the low penetration depth, or if a triaxial residual stress state with σ 33 ≠0 of opposite signs is present in the phases that compensate each other to macroscopic σ 33 =0 [6].In the present work, carbonitrided specimens made of steel grade 18CrNiMo7-6 were investigated in terms of RS distribution in both martensite and RA phases according to the standard sin

Influence of Tempering and Cryogenic Treatment on Retained Austenite and Residual Stresses in Carbonitrided 18CrNiMo7-6 Low Alloy Steel

“…By contrast, the residual stresses in any austenite surviving cryogenic treatment would likely be less compressive and/ or in tensile state. The low compressive and/ or tensile state in RA have been reported in the earlier work of the authors (Katemi et al, 2014b) in which RS in both retained austenite and martensite were measured. Okhi (2006) reports that such tensile stress in RA reduces the fatigue strength of the component.…”

Relationship between microstructure, and residual strain and stress in stainless steels in-situ alloyed by double-wire arc additive manufacturing (D-WAAM) process

“…The expansion of the structure upon transformation thus leads to the establishment of compressive residual stresses within the formed phase (martensite). As Katemi [47] observed, the larger the martensite fraction, the higher the compressive stresses. The lower the temperature at which the martensitic transformation takes place, the less additional tensile stresses will establish upon the rest of the cooling process, and thus the more these zones can be maintained in compression.…”

In-situ Observation of Retained Austenite and Residual Stress Evolutions during Tempering of carbonitrided DIN 1.6587 Alloy Steel

“…Furthermore, nitrogen stabilizes austenite and enables retaining up to 50 mass.-% of austenite after quenching. Such high amount of retained austenite influences not only the mechanical properties but also the magnitudes and distribution of residual stresses in both martensite and retained austenite in the case hardened layer (Katemi et. al, 2014).…”