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

This paper reports an investigation of the influence of carbonitriding conditions for 20MnCr5 low alloy steel. Three gaseous carbonitriding conditions were investigated based on different carbon and nitrogen potentials to attain varying levels of carbon between 0.62 and 0.93% mass, whereas for nitrogen between 0.19 and 0.26% mass at the surface. Analysis of retained austenite and residual stress distributions was conducted using X-ray diffraction technique. The effective case depth varied between 900 and 1200 µm. The case microstructures were characterized by varying proportions of retained austenite and martensite, while the core contained essentially bainitic microstructures. The maximum amount of retained austenite which occurred at a depth of 50 µm from the subsurface ranged between 30 and 70% mass and significantly influenced the level of surface micro-hardness whereas the core hardness remaining relatively constant at 450 HV1. High values of residual stresses in martensite phase were observed. The signs, magnitudes, distributions and location of maximum compressive residual stresses were highly influenced by the maximum fraction of retained austenite. Retained austenite of 30%, 50% and 70% mass at the surface lead to peak compressive residue stresses of -280, -227, and -202 MPa at depths of 555, 704, and 890 μm, respectively. Keywords: Carbonitriding, retained austenite, martensite, residual stress, XRD.

This work investigated the influence of tempering conditions coupled with cryogenic treatment on thermal stabilization of retained austenite and residual stress distributions in carbonitrided 18CrNiMo76 low alloy steel samples. The carbonitriding conditions were set to enable attaining surface carbon and nitrogen content of 0.87 and 0.34 mass.-percent respectively. After carbonitriding, some of the samples were subjected to varying tempering conditions followed by cryogenic treatment at -120 °C using nitrogen gas. Analysis of both retained austenite and residual stresses was conducted using X-ray diffraction. In the as-quenched state, carbonitrided samples contained 52 mass.-percent. Samples that were directly subjected to the cryogenic treatment after quenching retained only about 20 mass.-percent of austenite. Samples subjected to variant tempering conditions coupled with cryogenic treatment retained at least 30 masses.-percent of austenite. A thermal stabilization of retained austenite which increases with increasing temperature was identified. On tempering at 240°C for 14 hours retained austenite becomes unstable and decomposes to bainite leading to the low initial amount of retained austenite before cryogenic treatment. It can be concluded that the tempering process coupled with cryogenic treatment leads to an increasing hardness, to higher compressive residual stresses as well as to a shift of the location of maximum compressive residual stress toward the surface.

This paper investigates the evolution of retained austenite and residual stresses during and after tempering of carbonitrided 18CrNiMo7-6 low alloy steel carried out using in-situ X-ray diffraction technique. In this case, two carbonitriding treatments with different surface the retained austenite contents of 20 and 54 mass.-% are investigated. The tempering is carried out in a continuous heating mode to 650°C as well as in isothermal mode at holding temperature of 170, 240, and 300°C for 2 hours. During continuous heating at a heating rate of 10°C/min, the retained austenite started to decompose at 290°C. On isothermal holding at 170°C for 2 hours, the retained austenite remained relatively stable at 20 and 54 mass.-% while readily decomposed to less than 5 mass-% on holding at 300°C. On continuous heating, residual stress in martensite continuously relaxes and reaches full relaxation (0 MPa) at about 400°C. During isothermal holding, residual stresses in martensite are increasingly relaxed with increasing holding tempering. Further relaxation of residual stresses is observed during cooling whereas a cyclic variation of the residual stresses in the retained austenite could be determined. Keywords: Carbonitriding, retained austenite, residual stresses, tempering, in-situ XRD This paper investigates the evolution of retained austenite and residual stresses during and after tempering of carbonitrided 18CrNiMo7-6 low alloy steel carried out using in-situ X-ray diffraction technique. In this case, two carbonitriding treatments with different surface the retained austenite contents of 20 and 54 mass.-% are investigated. The tempering is carried out in a continuous heating mode to 650°C as well as in isothermal mode at holding temperature of 170, 240, and 300°C for 2 hours. During continuous heating at a heating rate of 10°C/min, the retained austenite started to decompose at 290°C. On isothermal holding at 170°C for 2 hours, the retained austenite remained relatively stable at 20 and 54 mass.-% while readily decomposed to less than 5 mass-% on holding at 300°C. On continuous heating, residual stress in martensite continuously relaxes and reaches full relaxation (0 MPa) at about 400°C. During isothermal holding, residual stresses in martensite are increasingly relaxed with increasing holding tempering. Further relaxation of residual stresses is observed during cooling whereas a cyclic variation of the residual stresses in the retained austenite could be determined.

Advancements in automotive technologies, Information and Communications Technology and renewable energy technologies have increased the use of lead acid batteries as a source of portable and rechargeable energy. This has considerably increased the number of spent batteries with adverse effects on the environment and human health; which calls for recycling of spent batteries. This work was conducted to investigate challenges facing the formal business of recycling spent batteries and potential manufacturers of new lead-acid batteries in Tanzania. The work involved collection of information from key stakeholders such as collectors, dealers, Tanzania Revenue Authority and recyclers. It was found out that about 2 million units of used batteries are available in Tanzania annually; weighing a total of about 8,440 tonnes. At the moment of conducting this work, only two recycling plants were in operation: Ok Plast ltd and Gaia Eco-Solutions (T) Ltd. The two operational ULAB recycling plants process about 6,000 tonnes per year. Despite the large number of spent batteries available annually, the amount received by the two recycling plants is far less than the number of spent batteries available in country. Gaia plant processes about 3,600 tonnes of spent lead batteries per year while OK-Plast is only about 1,500 tons/year. The study revealed existence of illegal exportation of ULAB which is against national Regulations and the Basel International Convention. A number of challenges and opportunities have been discussed. Despite of the challenges, the local ULAB recycling industry in Tanzania is encouraged to explore opportunities of manufacturing new batteries. A number of recommendations have been provided; however, enforcement of the export ban of used Lead Acid Batteries has been stressed.