Mechanisms and Modeling Comparing HB7 and A723 High Strength Pressure Vessel Steels

Abstract

HB7, an ultra-clean, high strength pressure vessel steel manufactured in France, is compared to A723 steel. This steel, suggested as an improved pressure vessel material is currently being proposed for critical applications, and will likely be used more frequently as design engineers discover its capabilities. This paper includes comparisons of strength, fracture toughness, fatigue properties and composition of the two steels, followed by an in-depth comparison and modeling of environmental cracking resistance, Bauschinger-modified residual stresses and fatigue lives. Results indicate that in all critical areas, with the exception of Bauschinger-reduced residual stress, the HB7 is superior to the A723 steel. Particularly for small amounts of autofrettage, near-bore residual stresses are reduced for HB7 steel compared to those for A723 steel at the same strength level. The greatest improvement of the HB7 over the A723 is in environmental cracking resistance. The HB7, when tested in concentrated sulfuric acid, exhibits five orders of magnitude longer crack incubation times and three orders of magnitude slower crack growth rates, when compared to A723 steel at the same strength level.

Design of two-pass swage autofrettage processes of thick-walled cylinders by computer modeling

“…The basic material properties are listed in Table 2. 16,29,37–43 The cannon barrel’s material nonlinear stress–strain curve with the Bauschinger effect is shown in Figure 2, in which the equations for the material model under loading are represented as where σ and ɛ are true stress and true strain, respectively. ɛ y (=5.1 × 10 –3 ) is the strain at the yield point, and σ y (=1068 MPa) is the yield stress.…”

“…The basic material properties are listed in Table 2. 16,29,[37][38][39][40][41][42][43] The cannon barrel's material nonlinear stress-strain curve with the Bauschinger effect is shown in Figure 2, in which the equations for the material model under loading are represented as…”

“…[8][9][10][11][12][13]16,21,27,30,33 However, deformation in a swage autofrettage process is localized and dynamic with the Bauschinger effect, which in turn affects the residual stress developed. 12,17,[21][22][23]30,[34][35][36][37][38][39][40][41][42][43][44][45][46] Furthermore, for a twopass swage autofrettage process, the deformation induced by the second pass is based on the deformation generated by the first pass and is deformation history dependent. All these add difficulty to the analytical solution of a complex material model and process like the two-pass swage autofrettage process.…”

“…Computer modeling based on finite element analysis (FEA) provides a powerful alternative for evaluating the residual stresses and optimizing the swage autofrettage process. 16,[22][23][24]30,33,35,38,44,45,[47][48][49][50][51][52] Nonlinear material models with the Bauschinger effect can be easily implemented. However, there exists no systematical analysis and optimization of the two-pass swage autofrettage process, especially applied to high strength pressure vessels.…”