Effect of Cryogenic Treatment on the Microstructure and Mechanical Properties of ARC- Welded D2 Tool-Steel Joints Fabricated from Recycled Leaf Spring Scrap

Abstract

Cryogenic quenching media have been identified as one of the quenchants suitable for the enhancement of the properties of steel during heat treatment, yet the scope of its application by manufacturing industries for the fabrication of structural components is limited. In this study, the effect of cryogenic treatment using liquid nitrogen on the microstructure, tensile strength, hardness, impact strength, fatigue strength, toughness of an Arc-welded D2 tool steel joints fabricated from a recycled leaf spring material were investigated. The choice of the leaf spring steel material was as a result of continuous failure of a leaf spring used on a water tanker welded and quenched in air under normal room temperature. Samples for each property test were cut from the recycled leaf spring material obtained from the water tanker and the joints were fabricated using Arc-welding. Chemical composition of the recycled leaf spring steel material was determined using optical emission spectrometry (OES) technique. Scanning electron microscopy (SEM) was used to determine the microstructure of the recycled leaf spring material before and after the cryogenic treatment. Sixteen samples (two samples for tensile strength, hardness, impact strength and fatigue strength respectively) were produced for the mechanical properties’ tests. Eight of the samples were quenched in air under room temperature while the other eight samples were further subject to cryogenic treatment by quenching in liquid nitrogen (at -196^OC) in a canister for 45 minutes.Results of the chemical analysis showed that the leaf spring material is a high carbon, high chromium cold worked tool steel material, a ledeburite. The microstructure of the test sample before the cryogenic treatment was highly defined by a massive volume fraction of hard carbides, an austenite. After the cryogenic treatment, there was evidence of elimination of this retained austenite into hard martensite precipitates ultra - fine secondary carbides and relieved micro – stresses across the weld zone. The tensile strength, hardness, impact strength, fatigue strength, modulus of elasticity and percentage elongation of the control samples were evaluated as 649.4MPa, 85.0HRB, 85J, 70MPa, 2020MPa and 32.2% respectively. For joints further subject to cryogenic treatment the tensile strength, hardness, impact strength, fatigue strength, modulus of elasticity and percentage of elongation were obtained as 1063.9MPa, 108.7.5 HRB, 100J, 95.5MPa, 2960.55MPa and 36.85%. From the results it is clear that cryogenic treatment improved the properties of the leaf spring material. Thus, machine components made from high carbon steel can be subject to cryogenic treatment in order to enhance their properties.