The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusTR and the nondimensionalized crack initiation fracture toughnessJIc is described and shown to yield a good fit to experimental toughness data for a wide range of steels.

/pdf/on-macroscopic-and-microscopic-analyses-for-crack-initiation-55tan5fazh.pdf

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

The ductile fracture of metals: A microstructural viewpoint

This study aims at correlating microstructure and Charpy impact properties in high-toughness API X70 and X80 line-pipe steels. Three kinds of steels were fabricated by varying alloying elements and hot rolling conditions, and their microstructures and Charpy impact properties were investigated. In addition, their effective grain sizes were characterized by the electron back-scatter diffraction (EBSD) analysis. The Charpy impact test results indicated that the steels rolled in the single phase region had the higher upper shelf energy (USE) than the steel rolled in the two phase region because their microstructures were composed of acicular ferrites. In the X80 steel rolled in the single phase region, the decreased energy transition temperature (ETT) could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. Thus, it had excellent mechanical properties in yield and tensile strengths, absorbed energy, and transition temperature, except in ductility.

Correlation of microstructure and charpy impact properties in API X70 and X80 line-pipe steels

The AA 5083 alloy was deformed in torsion in the ranges 300–500°C and 01–1 S−1 to a strain of 5 The flow stress as a function of strain rose to a maximum value then gradually decreased towards a steady state and had a dependence on temperature and strain rate of the traditional form with an activation energy higher than that for pure Al Optical microstructures of specimens quenched after working ranged from elongated grains at 300°C to recrystallized grains at 500°C Transmission electron microscope subgrain structures from all conditions of working were more recovered at higher temperatures and lower strain rates The subgrain diameters are smaller than those in commercial Al mainly as a result of the increased density of particles and this is in agreement with observations in extrusions Since the degree of recovery is not substantially less than in commercial Al, one is led to confirm the theory that the dynamic recrystallization is caused by particles of <06 μm

Hot deformation and dynamic recrystallization of Al-5Mg-0.8Mn alloy

The micromechanisms of cleavage fracture in ferritic steels are reviewed. Mechanisms of crack nucleation by dislocation interaction are compared with those involving the formation of stable microcracks. It is concluded that cleavage in steels containing discrete carbide particles is nucleated by microcracks formed in the carbide particles when fracture obeys a critical tensile stress criterion. The cleavage fracture stress in these steels is seen to depend largely upon the size of the cracked carbide particle. No micromechanisms have been established for cleavage in martensitic or low-carbon bainitic steels but the packet size seems to control cleavage fracture in these materials.Two models of the relationship between the micromechanism of cleavage and fracture toughness are discussed in some detail. The statistical competition between different sized crack nuclei at a loaded crack tip can be represented by the requirement that the fracture stress be exceeded over a microstructurally determined ch...

D. A. Curry

Papers

Cleavage micromechanisms of crack extension in steels