Novel Austenisation Procedure for Quenched and Tempered 51CrV4 Spring Steel

Abstract

VDL Weweler has developed a novel heat treatment approach for their trailing arms, which eliminates the traditional austenisation step and instead capitalises on the inherent heat used for hot-forming to austenise the steel before quenching. This not only saves energy, but surprisingly also improves the lifetime of the products. The underlying reasons for this improvement in fatigue life are the central focus of the present study. Two trailing arms were used for this study, one from the traditional ‘Reference’ process, and one from the new ‘Gasloos’ process. Both were produced from the same heat of raw 51CrV4 steel bars that are inductively heated to 1200°C before hot-forming, after which Gasloos is immediately quenched to 180°C. Reference is left to cool to 600°C before being reheated in a gas oven at 900°C for one hour and is finally also quenched to 180°C. After quenching, both are tempered at 450°C for one hour, quenched to room temperature, and shot peened. Reference also underwent the usual painting procedure, Gasloos was collected before this. Both underwent fatigue bending tests, where the Gasloos trailing arm lasted about seven times longer. Fracture surfaces and unfatigued parts were collected from both trailing arms for analysis. Since it was obvious early on that the Reference trailing arm has a partially intergranular fracture, a literature survey was performed to assess the possibility that grain boundary segregation and embrittlement would occur. Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray Spectroscopy (EDS) was used to perform fracture and microstructural analysis, as well as to measure grain boundary segregation. X-Ray Diffraction (XRD) analysis was used to measure the depth profiles of the residual stress to assess the effectiveness of the shot-peening. Samples for XRD were cut using Electronic Discharge Machining (EDM). In both trailing arms, cracks initiated on slag inclusions present from the steelmaking process. Reference initiated on an inclusion 10μm in size, while Gasloos initiated on an inclusion 45μm size, 4.5 times bigger. Slag inclusions are on average 16.09μm in size. The Reference fracture surface is 16% intergranular, while Gasloos is fully transgranular. Carbide density on the PAGB is more than twice as high in Reference, while overall carbide density is 10% higher in Gasloos. Solute segregation to the PAGB could not be
measured with EDS. XRD analysis shows that the compressive residual stress from shot-peening is 1.5 times higher and twice as deep in Gasloos compared to Reference. This is due to the oxide scales, containing 17.49 weight% oxygen, that comprise the surface of Reference, making it very brittle and filled with voids. Gasloos does not have these oxide scales at the surface.