Exhaust system piping made by hydroforming: relations between stresses, microstructure, mechanical properties and surface

Abstract

The subject of research is car exhaust system piping made of chromium–nickel steel of grade AISI304L with a unique, complex shape that was obtained by hydroforming technology. The purpose of the research was to determine the relation between the microstructure features, surface condition, hardness and the stresses on the external surface as determined by the sin2ψ X-ray method. We found that the stresses were tensile and correlated with the steel hardness, i.e. they were greater where the hardness was higher. Moreover, longitudinal stresses showed a relationship with pipe wall thickness, while circumferential stresses did so only partially. According to our data, the greatest value of stress determined in the pipe amounted to 290 MPa, and was close to the yield point of the strain hardened 304L steel. As depicted via XRD and SEM examination, the pipe stress level and hardness were influenced by the transition γ→α’. Furthermore, in the region of higher stress and hardness, the amount of martensite was 10 vol.%. We also noted that the pipe’s outer surface when subjected to friction against the die shows lesser roughness compared to its inner surface upon exposure to water under pressure.

Effect of Hydroforming Drawing Cups on Thickness Variation and Surface Roughness

“…The roughness is increased at regions (C and D) because there is no contact between die and cup and due to the high deformed ratio. These results agree with Kucharska and Moraczyński [41] in exhaust system piping hydroforming. Region E has a high deformed ratio and simultaneously has a full contact between the die and cup surfaces.…”

“…It was showed that the side of the test specimen which is directed to the mold has a lower roughness than the membrane side. Kucharska and Moraczyński [41], objected to determine the mechanical properties of the exhaust system piping, such as residual stress and hardness, as well as its microstructure and wall thickness reduction.…”

Investigation of local properties of the Fe-Si alloy subjected to mechanical and laser cutting

“…For this reason, measurements were made using a linear aperture with a beam crosssection of 1 × 5 mm. As practice has shown, the applied linear aperture, averaging the stress results, turned out to be more useful compared to the point aperture, which is usually used in stress measurements in metals with smaller average grain size [29]. In the case of the tested M300-35 material, the point aperture "generates" results that are very dependent on the selected surface fragment (selected grain).…”

Formability Prediction of Laser-Welded Stainless Steel AISI 304 and AISI 430

“…The service life of the exhaust system parts on the inside is affected by changes in temperature, in salt concentration, and in concentration of the alkaline and acid condensates. Condensates such as sulfurous acid, sulfuric acid, and low levels of hydrochloric acid, and critical pH values cause corrosion of the exhaust system parts on the The corrosion resistance of these steels is guaranteed by the chromium content, which accounts for at least 10.5% [5][6][7]. When heated, chromium forms a protective coating of chromium oxide, which delays further oxidation.…”

“…Among other things, emissions standards require that the individual exhaust system parts (manifold, converter, resonator, and muffler) are exhaust-gas-tight throughout the vehicle's lifetime. The outer parts of these components are produced via processes of continuous pipe bending, conventional cold forming or hydroforming, and then they are joined into a single whole by edging or seam welding [7,8]. The edge joint is made of four overlapping layers of material stretching over no less than 8 mm.…”