Microstructure and Corrosion Properties of Modified Low Carbon Martensitic Stainless Alloys Deposited by Laser Metal Deposition

Abstract

Compared to conventional arc welding processes, laser metal deposition is a low heat input, low dilution welding process. Weld overlay of thin wall components can be done successfully with low distortion and low dilution, resulting in the proper chemical composition of the weld overlay. Refurbishment of cast steel and cast iron components can be done with greater success resulting in a significant lower tendency towards porosity, weld metal solidification cracking and heat affected zone cracking. Accurate control of welding parameters results in highly repeatable weld deposition. Positional welding can be done without difficulty due to the relatively small spot size. Laser metal deposition with powder as consumable is a non-contact process and allows for weld overlay across surface discontinuities. Residual magnetic effects in the substrate do not influence laser metal deposition in terms of arc wander as compared to arc welding processes. Undercut in the weld toe does not occur with optimised laser material processing parameters and weld repair on final machined surfaces can be done successfully. Due to the non-ionising radiation, porosity formation due to nitrogen absorption is not possible. The same is expected to apply to hydrogen and as a result, the risk for hydrogen cracking is dramatically reduced for chromium-molybdenum-vanadium steels. This study was based on three experimental martensitic deposits containing between 10.5 and 14% chromium, 1 to 6% molybdenum and 0 to 11% cobalt, with the steels with a higher molybdenum and cobalt content containing less carbon. The nickel content was constant at 5%. Nominal chemical composition of the low carbon (< 0.03%) martensitic stainless steel alloys were: 14Cr-5Ni-1Mo (alloy B), 11Cr-5Ni-3Mo-5.5Co (alloy C), 10.5Cr-5Ni-6Mo-11Co (alloy D). 316L austenitic stainless steel, with nominal chemical composition 18Cr-10Ni-2.5Mo (alloy A), served as the reference alloy for pitting corrosion resistance. Laser metal deposition of low-carbon martensitic stainless steel with addition of 1, 3 and 6% molybdenum was shown to produce fully martensitic microstructures in the as-welded condition. Rapid solidification of the weld pool suppressed the formation of delta ferrite and resulted in refined microstructures and improved mechanical properties. Fully martensitic microstructures were demonstrated by bulk X-ray diffraction. The absence of delta ferrite in the as-welded condition was confirmed by electron back-scatter diffraction for all three steels.