Effect of electroslag refining on the fracture toughness and fatigue crack propagation rates in heat treated AISI 4340 steel

The AISI 4340 steel has been electroslag refined and the improvement in mechanical properties has been assessed. Electroslag refining (ESR) has improved tensile ductility, plane strain fracture toughness, Charpy fracture energy, and has decreased fatigue crack growth rates. The K_IC values for the ESR steel are nearly twice those estimated in the unrefined steel and higher than those obtained in the vacuum arc remelted steel. Fatigue crack growth rates in region I and in region III are found to be decreased considerably in the ESR steel, while they are unaffected in region II. Measurements on heat treated samples have shown that the ESR steel has a better response to heat treatment. Both the suggested heat treatments namely austenitizing at 1140–1470 K as well as the conventional heat treatment of austenitizing at 1140 K have been followed. The improvement in the mechanical properties of ESR steel has been explained on the basis of removal of nonmetallic inclusions and reduction in sulfur content in the steel.     

Fracture behaviour of dynamically loaded ship building plate material

The fracture toughness values of ship building mild steel measured over a temperature range ? 196°C to 28°C and crack tip strain rates ranging from 10^?5/sec to 10^?1/sec are examined in the light of the models recently proposed by Malkin and Tetelman. The effect of a change in inclusion morphology brought about by electroslag refining on the fracture toughness of the steel is also evaluated. It is found that the stress-induced fracture criterion ofthe model applies for the case where the ratio $\text{σ}{}^1{}_{\mathrm{f}}\text{σ}{}_{\mathrm{YS}}\text{? 3.94}$. This ratio is independent of the strain rate. In the strain induced fracture region of the model, the critical strain near the crack tip, ?_f(Rβ) is a function of the yield stress irrespective of temperature and strain rate. Electroslag refining reduces significantly the size and volume fraction of the inclusions and changes their shape from prolate ellipsoid to spherical. Apparently the electroslag refining does not improve fracture toughness significantly if the fracture toughness of the as received material measured with the major axis of the inclusions perpendicular to the crack front, is taken as a basis of comparison.     

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions. It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low-temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature. An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.     

A theory for fatigue crack propagation

A new continuum mechanics model is developed for predicting fatigue crack propagation rates using a fracture mechanics approach. The model demonstrates the critical dependence of fatigue crack growth on the fatigue ductility exponent, the fatigue ductility coefficient, the elastic modulus and the fracture toughness; it is related to the stress intensity range, implying that fatigue crack growth is critically dependent upon the condition at the tip of the crack.Four materials are studied, namely a creep resistant stainless steels, FV535; a $\text{2}\text{1}\text{2}$ per cent nickel-chromium-molybdenum direct hardening steel, 2S96D; a nickel base heat resisting alloy INCO 901; and a ferrous alloy containing titanium carbide in a medium alloy tool steel matrix, known as Ferrotic C.The developed model provides a means of predicting crack propagation rates based on mechanical properties, and the simplified model provides a fundamental basis for a more general form of the Paris relationship.     

Temperature effects on fatigue crack growth in polycarbonate

The fatigue behaviour of a high strength thermoplastic, polycarbonate, has been investigated as a function of temperature. Fatigue crack growth properties were measured in the temperature range of 100 to 373 K and were analysed using a fracture mechanics approach. Fatigue behaviour was found to be related to the fracture toughness of the material. This correlation with fracture toughness was used to develop an empirical model based on the toughness for describing the effect of temperature on fatigue crack growth, and to consider fatigue in terms of the secondary losses of the polymer.     

Comparative cavitation erosion test on steels produced by ESR and AOD refining

Cavitation erosion studies of steels produced by Electroslag Refining (ESR) and Argon Oxygen Decarburization (AOD refining) have been carried out. The experiments were conducted using the modified ultrasonically induced cavitation test method. Erosion rates were measured and the morphology of damages under cavitation action was studied by scanning electron microscopy and optical microscopy techniques. The present work is aimed at understanding the cavitation erosion behaviour of electroslag refined steel (ESR) compared with the steel produced by Argon Oxygen Decarburization (AOD refining), commonly used in the production of hydraulic machinery parts (Pelton blades). The results exhibited lower cavitation rate of ESR steel compared with AOD steel, as a consequence of its better mechanical properties and homogeneous and fine-grained microstructure.     

Assessment of mechanical behaviour,microstructure and critical conditions of welded commercial purity titanium

An assessment of microstructure and mechanical properties of weldments in rolled and forged versions of commercially pure titanium has been undertaken. A comparison is also made between tungsten inert gas (TIG) and electron beam (EB) weldment properties.Fracture and fatigue behaviour of weldments have been investigated experimentally by fracture toughness and fatigue crack growth studies employing notched three point bend specimens. The implications of the data produced are expressed in a design philosophy which enables an estimate of critical defect size to be made based on either stable or unstable crack growth or plastic collapse. The conjoint action of fatigue and stable crack growth is also considered.     

氢蚀温度对20G和15CrMo钢疲劳行为的影响

研究了氢蚀温度对２０Ｇ和１５ＣｒＭｏ钢常规力学性能与疲劳性能和机制的影响以及应力比对氢蚀后２０Ｇ钢疲劳性能的影响结果表明，氢蚀后２０Ｇ钢的疲劳行为受氢蚀对材料损伤程度和氢蚀导致粗糙的裂纹途径造成闭合效应增加两因素联合作用；氢蚀后１５ＣｒＭｏ钢脱碳严重，其疲劳行为仅受脱碳对材料损伤作用的影响；应力比对２０Ｇ疲劳性能也有很大影响     

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

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 (ΔK) were determined to obtain the threshold value of stress intensity range (ΔK_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.     

Fatigue crack growth characteristics of rotor steels

Room temperature fatigue crack growth rate data were generated for Ni-Mo-V (ASTM A469, Cl-4), Cr-Mo-V (ASTM A470, Cl-8) and Ni-Cr-Mo-V (ASTM A471, Cl-4 and a 156,000 psi yield strength grade) rotor forging steels. Testing was conducted with WOL type compact toughness specimens and the results presented in terms of fracture mechanics parameters. Data show that the Ni-Cr-Mo-V steels exhibit slower fatigue crack growth rates at a given stress intensity range (ΔK) than do the Ni-Mo-V steels. In addition, the Cr-Mo-V steel was found to exhibit slower growth rates than the other alloys at ΔK levels below 40 ksi √in but somewhat foster rates at ΔK levels in excess of 45 ksi √in. The fatigue crack growth rate properties of the alloys studied conform to the generalized fracture mechanics crack growth rate law where da/dN = C₀ΔK^R. It was noted that the fatigue crack growth rate parameters n and C₀ tend to decrease and increase, respectively, with increasing material toughness, K_ic.     

Influence of grain refining elements on mechanical properties of AISI 430 ferritic stainless steel weldments – Taguchi approach

The effect of grain refining elements such as copper, titanium and aluminum on transverse tensile strength, ductility, impact toughness, microhardness and austenite content of AISI 430 ferritic stainless steel welds through Gas Tungsten Arc Welding (GTAW) process in as-welded condition was studied. Taguchi method was used to optimize the weight percentage of copper, titanium and aluminum for maximizing the mechanical properties and austenite content in the weld region of ferritic stainless steel welds. Based on Taguchi orthogonal array the regression equations were developed for predicting the mechanical properties of ferritic stainless steel welds within the range of grain refining elements. The observed mechanical properties and austenite content have been correlated with microstructure and fracture features.     

TA15钛合金两类组织对疲劳性能和断裂韧度的影响

研究了TA15钛合金片状和双态两种典型组织对疲劳性能和断裂韧度的影响,结果表明:在S-N曲线的高应力区,双态组织的疲劳强度高于片状组织;在低应力区,情况则相反,且片状组织的疲劳极限(656MPa)高于双态组织(565MPa).片状组织的疲劳裂纹扩展速率低于双态组织,且断裂韧度K1C高于双态组织,即片状组织的损伤容限性能优于双态组织.     

Relation between fatigue crack initiation and propagation,toughness and microstructure in large steel blooms for automotive plastic molds

Molds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due to their dimensions, the heat treatment produces mixed microstructures, continuously varying with the distance from the quenched surface, at which fracture toughness and fatigue behavior are not well known; fracture toughness is generally lower than that corresponding to a fully quenched and tempered condition. The response of the mold to defects and stresses applied during service depends on steel properties, that in turn depend upon the heat treatment and the microstructure.A survey of the mechanical properties of some commercial blooms was carried out by using three point fatigue bending tests on notched samples to evaluate the threshold behavior and the crack growth behavior by ΔK-decreasing and ΔK-increasing methods. The samples were obtained from different depths of the blooms. The relationship between mechanical properties, fracture surfaces and microstructure was also investigated.     

Effect of pearlite interlamellar spacing on fatigue property of a wheel steel

Different pearlite interlamellar spacings of 0.54 % carbon (C) wheel steel were obtained by heat treatment, and the influence of interlamellar spacing on fatigue behavior of the steel was studied through conducting the staircase method fatigue testing and decreasing/increasing stress intensity factor range (ΔK) fatigue crack growth tests. The results indicate that the fatigue endurance limit and fatigue threshold with smaller interlamellar spacing are higher than those with larger one, which can be well explained by dislocation slipping theory. Furthermore, the fatigue crack growth rates in the near-threshold and Paris regions were found to reduce with decreasing the interlamellar spacing. The decreased growth rate is attributed to the deflected crack path induced crack closure effect, as evidenced by fatigue steps on the fatigue fracture surface. The present results show how to enhance the fatigue property of wheel steel through refining the pearlite interlamellar spacing.     

Bruchmechanische Eigenschaften von metastabilen Austeniten

Fracture Mechanical Properties of Metastable Austenites The effect of a martensitic tranformation at the crack tip on fracture mechanical properties was investigated with FeNiAl-model alloys. Transformable austenite and martensite obtained by deep-cooling showed a completely different behaviour. The martensite has high yield stress, normal dependence of fracture toughness of specimen diameter, and a low threshold for the start of fatigue crack growth. Characteristic for the metastable austenite is a high work hardening ability (at a low yield stress) by stress-induced martensitic transformation in a zone at the crack tip, which is surrounded by untransformed austenite. This leads to a compressive internal stress, which impedes crack growth. A consequence is a high fracture toughness, which even increases with specimen thickness, and a very high threshold value for fatigue crack growth. Localized stress induced martensitic transformation associated with a positiv volume change can explain the anomalous fracture mechanical properties of the alloys in the metastable austenitic state.     

Life extension technology for steam pipe systems—I. Development of material properties

Material properties of A106B low-carbon steels were developed for life prediction analyses of steam pipes operated at elevated temperatures but in the sub-creep temperature range. Tensile, fracture toughness, fatigue crack growth rate and low-cycle fatigue properties were obtained on the piping steel at 24°C (75°F) and 288°C (550°F). The latter temperature corresponded to the highest operating temperature of nuclear plant steam piping. Increasing the test temperature from 24°C (75°F) to 288°C (550°F) decreased the yield strength and fracture toughness of the steel. Fatigue crack propagation rate properties at 24°C (75°F) and 288°C (550°F) were found to be comparable.

In the low-cycle fatigue tests, below a strain amplitude level of approximately 0.5%, cyclic softening was observed, while at higher strain levels, cyclic hardening was present. Based on the results of tensile and incremental-step fatigue testing, the strain-life curve was predicted. The predicted strain-life curve was found to be in agreement with the experimental result.

The fracture surfaces of fracture toughness specimens showed ductile fracture, while striations were observed on those of fatigue crack growth specimens. Fatigue striations were also observed on the fracture surfaces of low-cycle fatigue specimens. Fatigue initiation was associated with inclusions. It was shown that plastic straining in A106B steel could be detected by acoustic emission and by monitoring the eddy current response. These nondestructive evaluation techniques exhibit possibilities for in-situ monitoring of fatigue deformation.

While the development of material properties for the life prediction assessment of steam pipes is included in Part I of this paper, the establishment of a quantitative life prediction methodology and inspection criteria is contained in Part II. The developed life prediction methodology quantifies the effects of operating parameters on the remaining life of steam pipes using the material properties obtained in Part I.     

Mechanical properties of In-Situ Composites

In this paper, author's results of several years of research work on the mechanical properties of directionally solidified (In-Situ) composites are reviewed. Alloy systems investigated were the fibrous Al-Ni, Fe-MnS and the cobalt base superalloy Co-Cr-C and the lamellar Al-Cu and Co-W. The mechanical behavior of the above systems were studied under both static and dynamic loadings. Static loading involved tension, compression and 3-point bending and the dynamic loading involved rotating bending fatigue, fatigue crack propagation and strain controlled fatigue. It was found that the tensile fracture stress and toughness and the ultimate compressive stress were generally enhanced by increasing growth rate and/or temperature gradient. However, at very high growth rates, the properties were found to decrease due to misalignment of the structure. Models were suggested to describe the static behavior of the composites investigated. Good agreement was found between the model predictions and the experimented results which indicate that the static properties are structural sensitive. On the contrary, the fatigue life of the Al-Al₃ Ni was insensitive to structural changes caused by varying the growth rate. The fatigue crack propagation response of the Co-Cr-C composites was found to follow the Paris Erdogan relation. Examination of the fracture surface confirmed a brittle mode of fracture with fiber cleavage and matrix shearing to link up fiber breaks.     

The effect of sulphide inclusions on fracture toughness and fatigue crack growth in 12 wt% Cr steels

The variation of plane strain fracture toughness and fatigue crack growth rates with sulphide inclusion content has been examined for four different sulphur levels in a 12 wt% Cr stainless steel. Charpy impact tests have also been conducted and the experimental results are correlated with the volume fraction of inclusions and the inter-inclusion spacing. The availability of the experimental results for the case of steam turbine blading is discussed with a hypothetical example problem.     

Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti‐6Al‐4 V

Experimental investigation was conducted to evaluate the fracture toughness and fatigue crack growth characteristics in selective laser‐melted titanium 6Al‐4 V materials as a follow‐on to a previous study on high cycle fatigue. For both the fracture toughness and crack growth evaluation, the compact tension specimen geometry was used. It was found that the fracture toughness was lower than what would be expected from wrought or cast product forms in the same alloy. This was attributed to the rapidly cooled, martensitic microstructure, developed in the parts. At low stress ratios, the crack growth rates were faster than in wrought titanium but became comparable at higher ratios. The fracture toughness appears to be higher when the crack is oriented perpendicular to the build layers. The difference in the average threshold and critical stress intensity values for the crack growth results for the three orientations was within the scatter of the data, so there was essentially no difference. The same was true for the empirically derived Paris Law constants. Residual stresses were likely to have overshadowed any variation in crack growth because of microstructural directionalities associated with build orientation.     

Microstructures and mechanical properties of an investment cast gamma titanium aluminide

Abstract

Investment castings have been produced in γ-TiAl of composition Ti–48Al–2Nb–2Mn (at.-%) using induction skull melting. The microstructures of the bars were studied in the as cast condition and after hipping and heat treatment. Heat treatment at 1200°C led to a near γ structure whereas treatment at 1350°C resulted in a nearly lamellar structure. However, a duplex structure was retained after treatment at 1300°C. Tensile, fracture toughness, and fatigue crack growth resistance tests have been carried out on specimens machined from different sized bars. The tensile properties increased with decreasing bar diameter but, conversely, both the fracture toughness and fatigue crack growth resistance improved as the bar diameter increased. It has been found that the fracture toughness and fatigue crack growth resistance in nearly lamellar structures were better than those in near γ structures, whereas duplex structures had intermediate properties. However, the tensile properties of duplex structures were better than both near γ and nearly α₂ /γ lamellar structures, with optimum values at 35 ± 5% α₂ /γ lamellae of ～400 MPa 0·2% proof strength, 470 MPa tensile strength, and 0·9% elongation.