A study on fatigue crack growth in dual phase martensitic steel in air environment

Abstract

Dual phase (DP) steel was intercritically annealed at different temperatures from fully martensitic state to achieve martensite plus ferrite, microstructures with martensite contents in the range of 32 to 76%. Fatigue crack growth (FCG) and fracture toughness tests were carried out as per ASTM standards E 647 and E 399, respectively to evaluate the potential of DP steels. The crack growth rates (da/dN) at different stress intensity ranges (∆ ∆K) were determined to obtain the threshold value of stress intensity range (∆ ∆K th). Crack path morphology was studied to determine the influence of microstructure on crack growth characteristics. After the examination of crack tortuosity, the compact tension (CT) specimens were pulled in static mode to determine fracture toughness values. FCG rates decreased and threshold values increased with increase in vol.% martensite in the DP steel. This is attributed to the lower carbon content in the martensite formed at higher intercritical annealing (ICA) temperatures, causing retardation of crack growth rate by crack tip blunting and/or deflection. Roughness induced crack closure was also found to contribute to the improved crack growth resistance at higher levels of martensite content. Scanning electron fractography of DP steel in the near threshold region revealed transgranular cleavage fracture with secondary cracking. Results indicate the possibility that the DP steels may be treated to obtain an excellent combination of strength and fatigue properties. Keywords. Dual phase (DP) steel; fatigue crack growth; fracture toughness; stress intensity range; intercritical annealing; crack closure.

Crack tip transformation zones in austenitic stainless steel

“…Also, the shear modulus G, the Poisson's ratio ν and the mode I stress intensity factor K I were introduced in eq. (11). To facilitate comparison between different results, Cartesian crack tip coordinates…”

“…in [1,2,3,4,5,6,7,8], is somewhat surprising since the hard martensitic phase is considerably more brittle than the ductile austenite. This transformation toughening and reduction of crack growth rate has been attributed to crack tip shielding and crack tip blunting [9], and crack closure due to roughening of the crack surfaces or due to the crack being subjected to compression by the dilatation in the transformed material [10,11]. In addition, several authors point out that the strongly dissipative process of martensitic phase transformation reduces the energy available at the crack tip for extending the crack [2].…”

Effects of Austenitising Conditions on the Microstructures and Mechanical Properties of Martensitic Steel with Dual Matrix Structure

“…6 and 7 According to Sudhakar et al [13], it was reported that the increase in strength obtained in DP steels is mainly attributed to (a) the load bearing capability of martensite in the DP structures, and (b) the fact that deformation in the ferrite phase is constrained by martensite. It has been further reported [11] that the yield strength and ultimate tensile strength of DP steels are linearly related to the amount of martensite, which increases with ICA temperatures.…”

Microstructure and fatigue crack growth behavior in tungsten inert gas welded DP780 dual-phase steel

“…For example, it was concluded that the hard martensite dispersed in the soft ferrite matrix can significantly influence the fatigue crack growth (FCG) rates [2,7,8]. The FCG rate decreased with an increase in martensite content.…”

“…The fatigue properties of dual phase steel have been studied by quite a few researchers [2,[7][8][9][10]. For example, it was concluded that the hard martensite dispersed in the soft ferrite matrix can significantly influence the fatigue crack growth (FCG) rates [2,7,8].…”