Assessing Hydrogen-Assisted Cracking Fracture Modes in High-Strength Steel Weldments Test results substantiate and extend the Beachem theory on hydrogen embrittlement BY S. A. GEDEON AND T. W. EAGAR ABSTRACT. The stress intensity that causes causes crack propagation as a function of then builds to very high values, which crack propagation in high-strength steel stress intensity for a specific material and adds to the applied external stresses. By weldments was quantified as a function of temperature. This relationship is then com­ applying Sievert's law, it is estimated that the hydrogen content at the crack loca­ pared to existing cracking mechanism the­ a steel with 5 ppm hydrogen would have tion. This relationship was used to assess ories. over 17,000 atmospheres pressure in the previously proposed theoretical hydro­ Previous literature concerning cracking voids at 20 °C (68 °F). However, several gen-assisted cracking mechanisms. In­ theories, stress intensity, determination experimental observations conflict with deed, it was found that the microplastic- and hydrogen content determination is this mechanism. Hydrogen embrittlement ity theory of Beachem can best describe briefly reviewed. can be eliminated by degassing even after how the stress intensity factor and hydro­ exposure to room temperature. The low gen content affect the modes of inter­ Hydrogen-Assisted Cracking Mechanism temperature of the degassing would not granular, quasi-cleavage and microvoid Theories be high enough to dissociate the diatomic coalescence fracture. hydrogen into monatomic hydrogen The results of theoretical studies of hy­ Implant test results were analyzed with which could diffuse out of the steel. Also, drogen embrittlement mechanisms pro­ the aid of fracture mechanics to deter­ the observation of hydrogen-induced posed by physical metallurgists have rarely mine the stress intensity associated with cracks growing on a free surface pre­ been applied to the field of welding. various modes of fracture. Diffusible weld cludes an internal pressure gradient as the Sawhill's study (Ref. 1) of HY-130 steel hydrogen results were analyzed with the driving force for crack growth. weldments, however, provides a good aid of a hydrogen distribution model de­ background for the ensuing analysis of the The adsorption theory of Petch and veloped by Coe and Chano to determine most often proposed hydrogen embrit­ Stables (Ref. 3) and further modifications the amount of hydrogen present at the tlement mechanisms. Even though the (Ref. 4) propose a lowering of the surface crack location at the time of fracture. problem of hydrogen embrittlement has free energy by hydrogen so that a crack The stress intensity and hydrogen con­ been studied extensively, no one theory can grow under a lower applied stress. tent responsible for the microvoid coales­ has become generally accepted. This theory has been criticized on the ba­ cence fracture mode have been quanti­ sis that the small but finite plastic defor­ The planar pressure theory, proposed fied for the high-strength steel used in this mation observed on hydrogen-induced by Zapffee (Ref. 2), is based on the study. The resulting relationship agrees fracture surfaces requires more energy decrease in solubility of hydrogen as the with the results of Beachem but extend his than could be explained by the adsorption temperature is lowered. The atomic hy­ theory to a wider range of hydrogen con­ theory. In addition, fracture surfaces indi­ drogen is postulated to reassociate into tents. cate rapid void formation and coales­ diatomic hydrogen in pores and micro- cence at low temperatures where rate of voids. The pressure of diatomic hydrogen Introduction surface migration would be negligible. A theory proposed by Troiano (Ref. 5) It is known that hydrogen-assisted suggests that hydrogen interacts with dis­ cracking is a complex function of the location pileups in areas of triaxial stress to amount of hydrogen, the stress, the tem­ KEY WORDS lower the cohesive strength. It is known perature and the microstructure of the that hydrogen will diffuse toward regions steel. The purpose of this study is to Hydrogen Cracking of high triaxial stress such as those associ­ quantify the amount of hydrogen that High-Strength Steei ated with a stress riser. When the con­ Fracture Modes centration reaches a given level, the inter­ High-Strength Welds action of hydrogen with dislocation arrays Stress Intensity ahead of the stress riser is postulated to be 5. A. GEDEON is a Materials Engineering Consul­ Crack Propagation sufficient to cause fracture. Troiano sug­ tant at Temav Center for Research, Venice, It­ Implant Test Results gests that this interaction is due to the va­ aly. T. W. EAGAR is a Professor at Massachuetts Diffusible Hydrogen lence electrons from hydrogen atoms en­ Institute of Technology, Cambridge, Mass. Microplasticity tering the unfilled "d" shells of the iron MIL-A-46100 Steel Paper presented at the 69th Annual AWS and modifying the repulsive forces which Meeting, held April 17-22, 1988, in New Or­ determine the interatomic spacing in tran­ leans, La. sition metals. WELDING RESEARCH SUPPLEMENT I 213-s the K,c of the final fractured area. Scanning electron microscopy (SEM) of H = 500 fractured implant specimens can be used 100 lo = 0.3 to determine the crack geometry. Speci­ mens with the same crack geometry as /© \ that studied by Daoud, etal., can then be used to determine the stress intensity fac­ 80 tor associated with that fracture. Determination of Hydrogen Content in the /© \\ Cracking Zone c 60 "™ 1 / / \ \1 o The diffusible hydrogen test can be O used to determine the amount of hydro­ c gen initially solidified into the weld pool. a) 40 11 fo ^^ However, since fracture in the implant specimens will occur sometime after the weld has cooled down, and some hydro­ ll i r& gen will have been lost by diffusion, these 20 results must be analyzed to determine the amount of hydrogen remaining in the ©\©\j cracking zone at the instant of fracture. The amount and distribution of hydro­ fig. 1 —Hydrogen 0 V i i distribution as a gen remaining in an implant specimen as a function of distance 0.6 0.8 1.0 function of time after welding can be es­ in the weld for r = timated with the aid of a model initially 0.011, 0.044, 0.10, developed by Coe and Chano (Ref. 13). 0.20 and 0.50. Top Surface Fusion Line Bottom Surface They used an iterative procedure using small time-at-temperature increments to calculate the effect of time on the hydro­ gen distribution. The results are presented Others have modified the planar pres­ this means will not behave in the same as hydrogen as a function of the nondi­ sure theory and the adsorption theory by way as hydrogen introduced by an actual mensional parameter 9. This value is de­ assuming that hydrogen atoms are trans­ welding process. fined as: ported to the void or crack tip as Cottrell e = Dt/|2 (1) atmospheres. Bastein (Ref. 6) has pro­ Quantification of the Stress Intensity in a posed that hydrogen atoms are carried where D is the diffusivity of hydrogen in Weld along by the movement of dislocations solid iron, t is time, and lo is the weld bead during plastic deformation. Thus, he rea­ Among the various testing methods for depth. A sample of their distribution plots sons, dislocation pileups at structural de­ assessing hydrogen embrittlement, the is shown in Fig. 1, which shows the fects will produce an oversaturation of implant test has become one of the most hydrogen distribution as a function of dis­ hydrogen, which will result in an increase popular for scientific investigations of the tance in the weld for various values of 9. in pressure which in turn produces triaxial cracking phenomenon in welds. This is A better plot for the purposes of this re­ stresses and embrittlement. Research by due to the fact that the stress, hydrogen search is shown in Fig. 2, which shows the Graville (Refs. 7, 8) supports the hypoth­ level and microstructure can be indepen­ hydrogen concentration as a function of 9 esis that hydrogen transport by disloca­ dently varied and controlled. Crack sus­ at various weld locations. tions to the site of crack initiation is a nec­ ceptibility using this test is typically defined It has been postulated that dislocation essary part of the embrittlement process. as the lower critical stress (LCS). The LCS sweeping will increase the actual amount Beachem (Ref. 9) has proposed a theory is the maximum stress at which fracture of hydrogen at the crack tip. The in­ of hydrogen-assisted cracking, based on a does not occur for an arbitrarily long pe­ creased solubility of hydrogen under an microplasticity mechanism rather than em­ riod of time (usually 1 to 3 days). applied axial tensile stress has been esti­ brittlement. He suggests that the hydro­ Fracture mechanics can be used to de­ mated to be 5 times higher than the nom­ gen in the lattice ahead of the crack tip termine the stress intensity associated inal solubility by Louthan, et al. (Ref. 14). assists whatever microscopic deformation with fracture in the implant specimens. Andersson (Ref. 15) used a finite element processes the microstructure will allow. Since the helical notch used on the implant technique to estimate that the hydrogen Thus, intergranular, quasi-cleavage or mi­ specimens is too blunt to use linear elastic in front of the crack tip is about 1.2 times crovoid coalescence fracture modes will fracture mechanics (LEFM), the crack initi­ the nominal bulk hydrogen value. Schulte operate depending on the microstructure, ation process is difficult to quantify. How­ and Adler (Ref. 16), using nuclear reaction the crack tip stress intensity, and the con­ ever, once hydrogen embrittlement oc­ analysis of deuterium distribution, deter­ centration of hydrogen.