Micromechanism of Cleavage Fracture of Weld Metals

doi:10.11900/0412.1961.2017.00145

Acta Metall Sin

2017, Vol. 53

Issue (11): 1427-1444 DOI: 10.11900/0412.1961.2017.00145

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Micromechanism of Cleavage Fracture of Weld Metals

Jianhong CHEN^1,²(

), Rui CAO^1,²

1 State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metal, Department of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China
2 Department of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China

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Jianhong CHEN, Rui CAO. Micromechanism of Cleavage Fracture of Weld Metals. Acta Metall Sin, 2017, 53(11): 1427-1444.

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Abstract

Cleavage fracture is the most dangerous form of fracture. Cleavage fracture usually happens well before general yielding at low nominal fracture stress and strain. Cleavage fracture is often spurred by low temperature and determines the toughness in the lower shelf temperature region. This paper describes a new framework for the micromechanism of cleavage fracture of high strength low alloy (HSLA) steel weld metals. Cleavage fracture not only determines the impact toughness in the lower shelf but also plays a decisive role on the impact toughness in the transition temperature region. The toughness is determined by the extending length of a preceding fibrous crack which is terminated by cleavage fracture. Three non-stop successive stages, i.e. crack nucleation, propagation of a second phase particle-sized crack across the particle/grain boundary, propagation of a grain-sized crack across the grain/grain boundary are explained. The "critical event" of cleavage fracture is emphasized which offers the greatest difficulty during crack formation and controls the cleavage process. The critical event indicates the weakest microstructural component and its critical size which specifies the local cleavage fracture stress σ_f for cleavage fracture. In toughness-study it is paramount important to reveal the critical events for various test specimens. Three criteria for crack nucleation, for preventing crack nucleus from blunting and for crack propagation are testified. An active region specified by these criteria is suggested where the combined stress and strain are sufficient to trigger the cleavage fracture. It can be used in statistical analyses. A case study, using the new framework of micromechanism for analyzing toughness of 8%Ni steel welding metals is presented to analyze the experimental results.

Key words: cleavage fracture fracture process critical event fracture criteria

Received: 24 April 2017

ZTFLH:

TG111

Fund: Supported by National Natural Science Foundation of China (Nos.51675255 and 51761027)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00145 OR https://www.ams.org.cn/EN/Y2017/V53/I11/1427

Fig.1 Appearance of cleavage crack^[8]
(a) metallographic section (b) fracture surface

Fig.2 Impact toughness-temperature transition curves^[8] (E_t—total charpy energy, T—test temperature, (a)—upper shelf, (b)—transition region, (c)—lower shelf, 1—material with the test temperature of -50 ℃ higher than the "transition temperature" notified by the red bar on curve 1, 2—the material with the test temperature of -50 ℃ near to the "transition temperature" notified by the black bar on the red transition curve 2, 3—the material with the test temperature of -50 ℃ lower than the "transition temperature" notified by the black bar on the blue transition curve 3)

Fig.3 Microscopic fracture surface
(a) dimples of ductile rupture fracture on the upper shelf
(b) cleavage fracture on the lower shelf^[8] (Arrow denotes the initiation origins of cleavage fracture, red region denotes critical facet of cleavage fracture)

Fig.4 Crack and fracture surface in the transition region
(a) a fibrous crack on the metallographic sec-tion
(b) a fibrous crack extending to certain length then transiting to cleavage fracture on the fracture surface
(c) stretch zone width (SZW), ductile cracking extension length (SCL), and the cleavage fracture distance (X_f) from cleavage fracture initiation origins to the blunted crack tip^[11]

Fig.5 Plots of the total impact energy with SZW+SCL (a), crack initiation energy with SZW (b), crack propagation energy with SCL (c) measured at various temperatures in welding metal specimens with various heat input^[11]

Fig.6 Schematics of normal stress (σ_yy) distribution along y direction in front of a notch root (a) and different lateral deformations (reductions) along y direction (b)

Fig.7 Schematic of stress distribution of normal stress ahead of a notch and a precrack (d_y—plastic zone size; σ—stress; σ_y—yield strength; σ_nom—nominal tensile stress; L—bending load, L_GY—general yielding bending load, n—hardening exponents; ε_p—plastic strain; the values of R are normalized by a parameter "δ_t=b" which is equal to the crack-tip opening displacement b)
(a) stress distribution of normal stress ahead of a notch without stress triaxiality
(b) stress distribution ahead of a notch with stress triaxiality^[8]
(c) stress distribution ahead of a precrack with stress triaxiality^[8]

Table 1 Chemical compositions of used materials (mass fraction / %)

Fig.8 Fracture surface of a C-Mn steel crack opening displacement (COD) specimen fractured at -130 ℃^[8]
(a) a crack nucleus (1.5 μm×2 μm, pointed by an arrow) initiated at a grain boundary carbide particle and propagated through boundaries between particle and contiguous ferrite grains 1 and 2
(b) grain 2-sized crack propagating into contiguous grains 3~7
(c) grain-sized cracks 1~7 form a cracking terrace
(d) the formed cracking terrace acting as a Griffith's crack around 100 μm×200 μm in size and triggering the global cleavage fracture of entire specimen

Fig.9 Schematics showing the cleavage fracture process^[8]
(a, b) nucleation and extension of a microcrack in a second-phase particle at the end of a dislocation pileup
(c) propagation of the just nucleated second-phase particle-sized crack into the matrix grain
(d) propagation of the grain-sized crack into contiguous grains and then throughout the entire specimen

Fig.10 The procedure for locating the crack initiation particle
(a) under low magnification trailing Chevron strips to the crack initiation region^[8]
(b) tracing back the river pattern strips on the fracture facets to find the origin particle of cracking initiation^[8]
(c) measuring its distance X_f from the notch root

Fig.11 Several types of entities of crack initiation^[8](a) plastic slip-induced cracking martensite-austenite (M-A) block(b) shear stress-induced fiber-loading cracking M-A flake(c) blocky M-A constituent-induced ferrite delamination(d) decohesion of an inclusion(e) cracking induced by carbide particle(f) cracking induced by pearlite column

Fig.12 Schematic manifesting of the mechanisms of crack initiation^[8]
(a) dislocation pileup-induced cracking in a second-phase particle (L—length of dislocation pileup, σ_e—equivalent stress inducing dislocation pileup, c—carbide second phase particle, r—length of carbide second phase particle cracking, σ_n—a normal stress which produced a crack nucleus in a second phase particle, σ₁₁ —applied tensile normal stress)
(b) surface shear stress (τ) induced fiber-loading cracking in a flaky M-A
(c) surface shear stress-induced decohesion of inclusion
(d) stress triaxiality-induced delamination of ferrite matrix near boundary of M-A block (σ_zz—tensile stress along crack tip, i.e. along the thickness)

Fig.13 A typical fracture surface showing the case of cleavage microcracking nucleated by stress triaxiality-induced delamination induced by a blocky M-A^[8](a) river-pattern strips originating from the blocky M-A that initiates the cleavage fracture(b) close look of the M-A with a thin foil of bainite matrix on its surface(c) M-A block and thin foil on it(d) carbon map by scanning Auger electron probe

Fig.14 Schematics of metallographic section cut perpendicularly to the fracture surface (a) and the crack retained close to the fracture surface (b)^[8]

Fig.15 Schematics of metallographic section for observing microstructure around the crack initiation site (a) and microstructure around the crack initiation showing large ferrite grains (b)^[8]

Fig.16 Retained crack in ferrite grain for notched specimen (a), retained crack in bainite packet for notched specimen (b) and retained cracks confined in second phase particles in precracked specimens (c)^[8]

Fig.17 Crack initiated in the coarsest grains in notched specimen^[8](a) coarse grain in the cleavage fracture initiation origins in four point bending specimen(b) coarse grain in the cleavage fracture initiation origins in Charpy toughness specimen

Fig.18 Measured σ_f and the histogram of grain size distributions for Ti-B weld metal (a) and C-Mn weld metal (b)^[8] (σ_{f max}—maximum σ_f, σ_{f min}—minimum σ_f)

Fig.19 Schematics of measuring σ_f (a) and measuring ε_pc (b)^[8] (σ_yymax—maximum tensile normal stress, σ_f—cleavage fracture stress, σ_yyi—tensile normal stress at initiation origin location, X_ma—the location of maximum tensile normal stress, ε_pc—critical plastic strain)

Fig.20 Distribution of σ_yy ahead of a crack tip^[8] (X—distance from precrack tip, σ_f′—assumed cleavage fracture stress)

Fig.21 Results of three point bending (3PB) precracked specimens unloaded at applied load of 6.86 kN^[8]
(a) precrack tip configuration observation
(b) FEM simulations of fracture behavior at precrack tip
(c) FEM calculations of stress and strain distributions (σ_m/σ_e—stress triaxiality, T_c—critical stress triaxiality, vertical line I—drawn from the intersection of the curve ε_p and the horizontal bar ε_pc, vertical line P—drawn from the intersection of the curve σ_yy and the horizontal bar σ_f (almost the same line through the intersection of σ_m/σ_e and the horizontal bar T_c), X_s—distance between I line and P line)

Fig.22 Results of 3PB precracked specimens unloaded at applied load of 9.80 kN^[8]
(a) precrack tip configuration observation
(b) FEM simulations of fracture behavior at precrack tip
(c) FEM calculations of stress and strain distributions

Fig.23 Results of 3PB precracked specimens unloaded at applied load of 12.74 kN^[8]
(a) precrack tip configuration observation
(b) FEM simulations of fracture behavior at precrack tip
(c) FEM calculations of stress and strain distributions

Fig.24 An active zone (with shaded area) satisfied with three criteria (a) (B—specimen breadth,

σ ?

—average stress), variation of the active zone width with applied load (P_f) (b) and the variation of the probability (P_ir) of fracture with applied load (c)^[8]

Fig.25 Microstructures of weld metals DM4-1 (a) and T5-21(b)^[8]

Table 2 Compositions, micro-parameters and mechanical properties of base metal (BM) and weld metals

Fig.26 Retained cracks in base metal (a) and DM4-1 (b)^[8]

Fig.27 Random measure retained crack and bainite colony sizes in base metal (a) and DM4-1 (b)^[8]

Fig.28 Histogram of bainite colony size in DM4-1 (a) and T5-21 (b)^[8]

Fig.29 EBSD maps (a, b) and grain misorientations (c, d) in DM4-1 (a, c) and T5-21 (b, d)^[8] (Blue line in Figs.29a and b presenting the misorientation angles larger than 15°)

Fig.30 Width of bainitic laths in DM4-1 (a) and T5-21 (b) measured by TEM^[20]

Fig.31 Locating the crack initiation site (a), stress and strain distributions in front of the notch root by FEM calculation (b)

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