The fatigue behaviour of an ultra-high strength steel (>1800 MPa) was evaluated to assess the potential of using laser cladding as a repair tool for such steels in aeronautical structural applications. AISI 4340 and AerMet 100 steel powder were used to clad over a grind-out region in an AISI 4340 steel substrate using a 2.5 kW ND:YAG laser. Post-clad heat treatment (PCHT) was also investigated. Results showed very poor tensile properties and significantly reduced fatigue life of the AISI 4340 as-clad with a very high hardness and brittle fracture in the clad and HAZ zone. Residual stress results showed a compressive residual stress in the clad region and tensile residual stress in the HAZ. Changing the alloy of the clad layer to AerMet 100 steel, as well as applying a PCHT process, showed promising results as the fatigue life was improved from that of the grind-out substrate.

Effect of laser clad repair on the fatigue behaviour of ultra-high strength AISI 4340 steel

In the current investigation AISI 4340 steel was laser deposited on a rolled mild steel substrate by Direct Metal Deposition (DMD) technology. The microstructural investigation of the clad was performed using optical and electron microscopes and X-ray diffraction techniques. The microstructure consisted of ferrite, martensite and cementite phases. Two types of martensite, lathe-type and plate-type, were observed in the microstructure. Decrease in microhardness values from the top layer to the alloy layer proves that the degree of tempering of the martensite phase increases in the same direction. The lattice parameters of the identified phases were found to be shorter than those reported in literature. The reported parameters in literature are from samples processed under equilibrium conditions.

Microstructural evolution of AISI 4340 steel during Direct Metal Deposition process

A study on the surface treatment of “Calmax” tool steel by a plasma transferred arc (PTA) process

This study discusses the effect of substrate preheating by an induction heater on the deposition characteristics of high-speed tool steel M4 powder. The powder is highly susceptible to cracking because of the very rapid heating and cooling rates induced by a high-energy laser during the deposition process and the residual stress evolution after deposition. Experimental investigations showed that a slower cooling rate can be induced by substrate preheating with the assistance of induction heating, which results in a coarser microstructure and a significantly longer spacing between cellular and dendrite arms. A high micro-hardness level may be obtained despite the microstructure evolutions occurring during the induction heating-assisted laser metal deposition because of the higher volume fraction of hard and stable carbides resulting from the increased holding time for precipitation. The tensile strength of the specimen prepared with substrate preheating increases. This effect may be attributed to the fine-grained martensite structure exhibited in the inter-diffusion and heat-affected zones in the sample that was not preheated. On the contrary, no distinct martensite structure was observed in the induction-heated sample. The fracture surface showed a typical brittle fracture, in which the crack propagated along the inter-dendritic boundaries. The results presented in this paper can be considered as guidelines for practical hardfacing applications that use high-alloy tool steel powders to achieve highly improved die and mold performances in terms of wear resistance and toughness.

Effect of substrate preheating by induction heater on direct energy deposition of AISI M4 powder

The effects of the heat-affected zone (HAZ) in AISI 4340 steel created by laser-surface alloying (LSA) on high-cycle fatigue behavior have been investigated. This research was performed by producing several lots of laser-processed AISI 4340 steel using different laser processing parameters, and then subjecting the samples to high-cycle fatigue and Knoop microindentation hardness studies. Samples of tested material from each lot were examined using scanning-electron microscopy (SEM) in order to establish the effects of laser processing on the microstructure of the fatigue-tested AISI 4340 steel. When these three techniques, microindentation hardness testing, high-cycle fatigue testing, and SEM, are combined, a mechanistic understanding of the effect of the HAZ on the fatigue behavior of this alloy might be gained. It was found that the HAZ did not appear to have an adverse effect on the high-cycle fatigue behavior of LSA-processed AISI 4340 steel.

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

The microstructure and microhardness of AISI 4340 steel melted by a continuous wave ĈO2 laser are examined. The parameters investigated are overlap between adjacent passes, laser power, and scanning speed. After melting, three different zones were observed in the treated surface corresponding to the melted zone, the heat affected zone, and the overlapping zone. The melted zone displayed a fine cellular structure, while the other two zones displayed fine martensite or bainite structures. The fine structure and the high microhardness values, which were measured in the melted zone, are evidence of the high quenching rates which follow laser melting. An increase in the scanning speed and a decrease in laser power resulted in afiner microstructure and higher microhardness, and a reduction in the amount of overlap improved the overall microhardness.MST/1038

M. Bamberger

Papers

Laser surface melting of AISI 4340 steel