Laser transformation hardening of tempered 4340 steel

Abstract

A CO2 laser with a fixed laser power of 1.8 KW was employed to harden the surface of some AISI 4340 steel specimens, with a scan rate from 5 to 10 mm/s. The influence of scan rates and tempering treatments of the alloy on the hardness profile and microstructure of the laserhardened zone was analyzed. Microstructures in the hardened zone consisted of mainly lath and twinned martensites. However, depending on the scan rate, autotempered martensite has also been found. In the transition zone of laser-treated specimens, partially dissolved carbides with austenite envelopes and/or austenite islands in a matrix of martensite were observed. The time required for complete carbide dissolution into austenite during laser treatment depended on the tempering conditions. A lower tempering temperature of the alloy produced a deeper hardened zone and a narrower transition zone in the hardness profile. A simple mathematical estimation of the hardness profile, based on the carbon diffusion distance in austenite, was performed. The calculated results are in reasonably good agreement with the measured hardness profiles and the microstructural observations in the laser transformation hardening process.

Effects of a laser surface processing induced heat-affected zone on the fatigue behavior of AISI 4340 steel

“…Over the next 200 m, which represents the HAZ, the hardness values remain constant until dropping to 400 Knoop at 400 m. At the end of the heataffected zone, the hardness values climb back up to 500 Knoop in the substrate and stay around this value for the remainder of the sample. The results of microhardness testing for Lots 1, 2, and 4 were similar to those reported by Shuie and Chen [22] who studied laser transformation hardening of AISI 4340 with a CO 2 laser.…”

Relationship between processing parameters, alloy atom diffusion distance and surface hardness in laser hardening of tool steel

“…Precipitation and carbide coarsening may occur even at high cooling rates experienced during laser transformation hardening [9,16,17]. This might have an effect on the hardness and properties of the quenched microstructure.…”

Numerical modeling and experimental investigation of the superficial layer of SKD61 steel during laser surface hardening

“…Therefore, the temperature of the surface layer during laser hardening of SKD61 steel should be higher than the austenized temperature of 1010°C in traditional quenching. Following this logic, the temperature of the surface layer should be high enough for quenching so that it will enable the complete transformation of austenite into martensite (Shiue and Chen, 1992;Bradley and Kim, 1988). Figure 10 presents the temperature distribution in the surface layer obtained by using the modified Ashby-Easterling heat-transfer equation (Eq.…”