Evolution And Control Of Stress During Laser Cladding Forming Of Martensitic Stainless Steel

Residual stress is one of the major issues affecting the large-scale commercial applications of laser cladding forming(LCF). The complex thermal cycling of the process unavoidably relates to the evolution of stresses and strains and lead to cracks and distortion. Moreover, the magnitude and distribution of residual stresses may strongly influence the service behaviors. For example, the tensile residual stresses are often considered to reduce the strength of the material, induce stress corrosion and short the fatigue life. Therefore, a valid approach to control the residual stresses is one of the most crucial problems to be solved in LCF.To investigate the influences of solid-state phase transformation on stress evolution during LCF process, a systematic study on the evolution of microstructure,material properties and stresses was done. Laws and mechanism of the effect of phase transformation on stress evolution were explored, and then some valid approaches to mitigate stress peaks and rearrange the stress distribution in favor of the product’s usage were given. A low transformation temperature martensitic stainless with high strength and high toughness was designed for LCF, which can be used for the remanufacturing of large centrifugal compressor.The temperature and history(microstructure) dependent material properties were obtained using Gleeble3500 thermal/mechanical simulator, MTS810 material testing system, DIL801 single-sample dilatometer, LFA 427-Laser flash apparatus and Setsys evolution. The material database, including transformation kinetics,thermophysical properties and mechanical properties, was then set up. The effects of solid-state phase transformation on material properties were explored. The Ms and chemical composition have been proved to be the main influences on the volume effects of phase transformation.The influences of external load, austenitization temperature and the cooling rate on the transformation kinetics were investigated. Martensite obtained in the process conditions of this study, and the cooling rates have small effect on microstructures. The austenitization temperature dominates the evolution of microstructure. The supersaturation increase with the increase of the austenitization temperature, and the solid–state transformation start temperature decrease. The cooling rates do have influence on the martensite start temperature, but the influence degree was small. The influences of external load on the transformation kinetics and transformation plasticity were investigated using Gleeble3500thermal/mechanical simulator. TRIP and stress induced martensitic transformationof high hardness martensitic stainless steel were investigated and further understand how the long extension was obtained. The existence of a strong texture along with the orientation of laser scanning shows that the choreography of atomic motions during transformation is particularly important.To investigate the influences of solid-state phase transformation on stress evolution during multi-pass LCF process, a 3D finite-element(FE)thermo-mechanical model considering phase transformation has been established.The influences of phase transformation such as mechanical properties changes,volume change and transformation induced plasticity(TRIP) are taken into account.Furthermore, the influences of high magnitude stress upon martensitic transformation characteristic temperature, transformation kinetics and TRIP are considered. The temperature and history(microstructure) dependent material properties were used in the present research.The stress evolution of single-pass and multi-pass cladding were investigated by both experiment and macro modeling. Four kinds of conditions are analyzed in comparation, three cases for phase transformation(one considering preheating, one considering high Ms) and one with no phase transformation at all. Stress measurement of Neutron diffraction, X-ray diffraction(XRD) method and blind-hole method are applied to deposited samples, and the measuring data are compared with the computational predictions.The results show that phase transformation has a dominant effect on the stress evolution, longitudinal residual stresses significantly reduced as a result of solid-state phase transformation. In addition, the effect of stresses on martensitic transformation temperature is important for accurate prediction of residual stresses(stress state after cooling of the clad to ambient temperature). Residual stresses are lower when the phase transformation temperature is reduced. The residual stresses of deposits can be controlled and minimized to a low level with uniform distribution, if most of the martensitic transformation in LCF is controlled to take place after laser cladding process. For low transformation temperature material, the influences of low temperature preheating on evolution are small. With the increase of the transformation temperature, the influences of preheating on residual stress increase.Tensile stress accumulation index(R L0.2? ? ?-(28)) was given to assess the degree of residual tensile stress accumulation during LCF.To reduce residual tensile stress of remanufactured impeller of large reciprocating compressor, this article discussed the factors that influence the Ms, the adjustment method of Ms by changing the chemical elements was given. A low transformation temperature martensitic stainless steel(Fe-Cr-Ni-Mo-Mn-Nb alloypowder) with high strength and high toughness was developed for the remanufacturing of large reciprocating compressor. The tensile stress level of the cladded coating is low. The mechanical property of as-deposited Fe-Cr-Ni-Mo-Mn-Nb alloy has a close performance to forging FV520 B.