Sandip Bhattacharya

Effect of Baking on Spot Welds of a Prestrained Ultra-Low Carbon Bake Hardening Steel Sheet

The effect of bake hardening (BH) treatment on the strength of spot-welded joints made on prestrained (PS) sheets of an ultra-low carbon (ULC) BH grade steel has been investigated using tensile-shear specimens. A series of these experiments has been carried out on spot-welds made on as-received and PS sheets; a part of the latter being subjected to BH treatment. Determination of hardness profile along the weldments, characterization of the microstructures, and substructures by optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM), as well as fractographic examinations are some necessary supplementary examinations. The results infer that the strength of the PS sheets increases by BH treatment, but this treatment does not exhibit any significant influence on the strength of the spot-welds made on PS sheets. The hardness profile along HAZ and fusion zone of the spot-welds made on as-received sheets remains almost unaltered for PS specimens or that subjected to BH treatment. The locations of failure around the spot-welds of as-received (AR), PR, and bake hardened specimens are similar in nature, and these failures usually originate from the HAZ-base metal interfacial region.

Breakage of Automotive Spring Washer during Twist Test: A Metallurgical Analysis

A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a screw or nut. The washer used in automotive engine component is of split or spring lock washer. A ring split at one point and bent into helical shape. The benefit of spring lock washers lies in the trapezoidal shape of the washer. When compressed to loads near proof strength of the bolt, it will twist and flattern. This reduces the spring rate of the bolted joint which allows it to maintain more force under the same vibration levels. This prevents from loosening. The steel washer is one of the critical safety component for automobile engine and its failure may cause severe safety issue. Before subjecting to manufacturing the washer undergoes different process lines (Wire Rod → Drawing → Annealing → Pickling → Flattening → Spring Washer → Single Spring Washer). After manufacturing of washer toughness test is done to find out the metallurgical soundness and surface quality of the washer. In this toughness test the washer in a vice with the split ends free and straight above the vice jaws, a 90° segment of the free end is gripped with a wrench and bent. Washers should withstand twist test through a 90° angle without signs of fracture. The present paper highlights premature failure of automotive washer which failed during twist test. From the analysis, it has been observed that the distribution of spheroidized carbide is more uniform in good samples than that of rejected samples. Hardness profile variation was observed between good and bad spring samples. Overall investigation does not indicate any problem related to supplied wire rod material quality as no wire breakage problem observed. Inhomogeneous annealed structure in flat wire seems to result into variation in washer performance during twist test.

Impact toughness of spot welds on prestrained interstitial free steel sheets

Abstract A series of impact tensile tests of spot welded joints made on interstitial free steel sheets have been carried out using a fabricated fixture. Spot welded samples were made from as received as well as prestrained steel sheets with the aim of examining the effect of prestrain on the impact tensile toughness of spot welds. The impact tensile toughness values of spot welds were found to be higher than their corresponding quasi-static toughness determined using tensile shear specimens. While the quasi-static toughness of spot welds decreases with increasing prestrain, the impact tensile toughness exhibits an opposite trend with prestrain within the range of investigation. The observed results have been discussed with considerations of specimen configurations, test procedure, microstructure, substructure, hardness profiles along the welds and post-failure examinations.