Hard Cladding by Supersolidus Liquid Phase Sintering: An Experimental and Simulation Study on Martensitic Stainless Steels

Martensitic stainless steels are suitable for diverse structural applications but degrade when subjected to wear-prone activities in service. To enhance their service life, the densification of high Cr, martensitic, X190CrVMo20-4-1 tool steel powder on two different martensitic stainless steel substrates via supersolidus liquid-phase sinter (SLPS) cladding was investigated. The objective was to assess the influence of the difference in compositions of the martensitic stainless steels employed as substrates on the interfacial diffusion, microstructure, hardness and bonding strength of the steel-to-steel claddings. Computational thermodynamics and diffusion simulations were employed to supplement experimental findings. Owing to interdiffusion, a M7C3 carbide-free, banded region exists in the X190 adjacent to the interface with the width dictated by chemical potential gradient of carbon. The hardness of the substrate was lower near the interface region because of carbon enrichment, which promoted the presence of retained austenite. An interfacial strength of 798 MPa was achieved with fairly ductile X190 matrix near the cladding interface as the fracture surface was characterized by mixed fracture modes of dimple rupture and cleavage with localized quasi-cleavage features. Experimental observations and computational simulations are in agreement. The implications of the SLPS cladding technique are discussed in the context of tool development.

Experimental investigation of fatigue failure in an ASTM A401 compression spring of a passenger car

This work outlines the fatigue failure in a failed helical coiled compression spring of a passenger car used as a commercial vehicle. As a failure investigation case study, the fractured left rear axle suspension spring of the car is thoroughly examined. The fractographic investigations included the visual examination, spectroscopy, microstructure analysis, SEM and micro-hardness testing. The foremost causes of failure have been identified. From the SEM fractograph, microvoid coalescence is observed at the grain boundaries on of the fractured surface which eventually resulted in intergranular dimple rupture. Residual stress is also measured on the fractured surface through two-dimensional (2-D) imaging using a Debye Scherrer ring, with the conclusion that the spring under investigation experiences micro-stress due to the variation of theintergranular spacing resulting from the vibrations induced by overloading, road conditions and prolonged use which eventually results in a rupturing of the compression spring.

Stress Corrosion Cracking Resistance of Cold-Sprayed Al 6061 Deposits Using a Newly Developed Test Fixture

The stress corrosion cracking (SCC) response of Al 6061 bulk deposits produced by high-pressure cold spray (HPCS) was investigated and compared to commercial wrought Al 6061-T6 material. Representative tensile coupons were stressed to 25%, 65% and 85% of their respective yield strength and exposed to ASTM B117 salt fog for 90 days. After exposure, the samples were mechanically tested to failure, and subsequently investigated for stress corrosion cracking via optical and scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). The results were compared to the wrought Al 6061-T6 properties and correlated with the observed microstructures. Wrought samples showed the initiation of stress corrosion cracking, while the cold-sprayed deposits appeared to be unaffected or affected by general corrosion only. Optical microscopy revealed evidence of stress corrosion cracking in the form of intergranular corrosion in the wrought samples, while no significant corrosion was observed in the cold-sprayed deposits. Fractography revealed wrought samples failed due to multiple mechanisms, with predominant cleavage and intergranular failure, but cold-sprayed samples only failed by ductile dimple rupture. The difference in SCC response between the differently processed materials is attributed to the documented benefits of the cold spray process, which includes maintaining fine grain structure of the feedstock powder and high density after consolidation, low oxidation, and work hardening effect.

Effect of Micro-Alloying with Boron and Niobium on Properties of Cr-Mn-Mo Steels

Economically alloyed steels for critical details of drill pipes have been studied. Effect of microalloying on structure and properties has been investigated. The article shows that boron and niobium additives change the structure and properties of Cr-Mn-Mo steels after quenching and high tempering. Methods of optical and electron microscopy have been used. Basic mechanical properties and impact strength of investigated steels are determined. Optical and electronic fractography has been carried out. The quantitative content of the ductile component is determined in steel fracture. It is shown that steel microalloying leads to a substantial structure refinement. This is due to the influence of niobium on the austenite grain value. An increase in the amount of carbide particles leads to structure refinement with an increased molybdenum content. Boron microadditives allow obtaining the tempered martensite structure throughout the product section. This provides an increase in both the strength and ductile properties. Combined microalloying of chromium-manganese-molybdenum steel with additions of boron up to 0.005 % and niobium up to 0.05 % makes it possible to increase the strength and reduce the tendency to brittle fractures significantly. The nature of the fracture becomes completely ductile. Distinct cleavage fracture surface feature “river patterns” are observed in unmodified steels. Сleavage facetes are large enough, it proves the presence of large grains in the steel. Microalloying changes the destruction mechanism, it becomes a ductile “dimple rupture”. An increase in the molybdenum content to 0.6 % makes it possible to obtain strength above 1100 MPa in microalloyed steel.

Theoretical and Experimental Investigation on Forming Limit for TNW700 Titanium Alloy

Forming limit is identified to evaluate the formability of sheet metal. The in-plane limit strains of sheets are plotted in a diagram with coordinates of major strain vs. minor strain. TNW700 titanium alloy is a high temperature resistant material. The products made of TNW700 can be used in a long serving period at 500°C, short time at 700°C. In this work, the forming limit of TNW700 will be investigated in theoretical and experimental ways. The experiment to test limit strains was carried out at 750°C under different loading paths. Marciniak – Kuczynski (M-K) model was calculated with Swift constitutive equations to predict the curves of limit strains. The effect of the groove angle on forming limit is that, the same angle on both sides of centerline determines the same FLC, and the limit points shift from left side to right side. The experiment shows that, the formability of TNW700 is not excellent, and it is lower than that of TC4 and TA15 at the same condition. The comparison shows that the curve predicted by M-K model is in a good agreement with that at plane strain, however higher than that in both sides. The fractographic observation shows that the fracture mode of TNW700 is dimple rupture.