Effect of cyclic loading on subsequent creep behaviour and its implications in creep–fatigue life assessment

Abstract

Evaluation of creep–fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep–fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr–1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clear improvement of creep–fatigue life prediction.     

Effect of W–Mo balance on long-term creep life of 9Cr steel

The effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture t_r exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in t_r with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe₂(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening.     

Effect of short-term data on predicted creep rupture life – pivoting effect and optimized censoring

Abstract

Fitting data to classical creep rupture models can result in unrealistically high extrapolated long-term strength. As a consequence, the standard strength values for new steel grades have frequently needed downward correction after obtaining more long-term test data. The reasons for non-conservative extrapolation include the influence of short-term data, which are easiest to produce but tend to pivot upwards the extrapolated values of creep rupture strength. Improvement in extrapolation could be expected by reducing this effect through model rigidity correction and censoring of very short-term data, but it may not be immediately clear how to justify the correction of particular models or censoring.

Analogously to the instability parameter in the minimum commitment model for creep rupture, a rigidity parameter correction (RPC) is introduced to assess the pivoting effect of creep rupture models for the purpose of reducing potential to non-conservativeness in extrapolation. The RPC approach can be used with any creep rupture model for comparing the model rigidity and the potential benefit from censoring short-term data. The correction itself will never introduce non-conservatism, regardless of the model. The RPC approach is demonstrated by analyzing an ECCC data set for cross-welded 9%Cr steel (E911).     

Effect of Ni interlayer on partial transient liquid phase bonding of Zr–Sn–Nb alloy and 304 stainless steel

Zr–Sn–Nb alloy and 304 stainless steel were joined by means of partial transient liquid phase bonding. The effects of Ni interlayer on the microstructure and properties of the joints were investigated. The reaction layers are formed in both joints and which are mainly composed of σ-FeCr layer, Zr(Cr, Fe)₂ + α-Zr layer and α-Zr + Zr₂(Ni, Fe) layer. The intermetallic compounds are compact relatively and cracks are formed in the reaction layer of the direct bonded joint. In the joint with Ni interlayer, many α-Zr phases dispersedly exist in the reaction layer and the thickness of the reaction layer is distinctly larger than that without Ni interlayer. As a result of lower residual stresses and wider crack-free reaction layer, the bonding strength of the joint increases by using Ni interlayer.     

Effect of grain boundary precipitation and δ-ferrite formation on surface defect of low nickel austenitic stainless steels

The present study elaborately discussed the effect of grain boundary precipitation and delta (δ) ferrite formation on surface defects of low nickel austenitic stainless steels. Several specimens were collected from different locations (i.e. centre and side) of cast slabs and rolled plates with variable nitrogen and carbon contents. The alloy segregation, precipitation and δ-ferrite formations were analyzed using several characterization tools. It was observed that the segregation of alloying elements along the grain boundaries is solely responsible for variation in precipitation and δ-ferrite formation among the specimens. Type of precipitation is mainly composition dependant. Relatively higher nitrogen and low carbon containing specimens show Cr₂N and/or Cr₃C_1.52N_0.48 rich continuous precipitation along the grain boundaries. On the other hand, decrease in nitrogen content along with the increase in carbon content results in discontinuous or cellular type of precipitation. Continuous grain boundary precipitates were found more detrimental compare to discontinuous or cellular precipitations. Micro-cracks are probably generated at the stress concentrated areas of precipitation–matrix interfaces and propagate along the interfaces or grain boundaries towards the stress free surface. Again, additional driving forces towards the surface crack formation and propagation are facilitated by the δ-ferrite and austenite interfaces during hot rolling.     

Effect of σ-Phase on the creep properties of Cr25Ni20 stainless steel

It is concluded from above that effect of σ-phase on creep properties depends on particle size and distribution. Fine σ-phase particles dispersively precipitated along grain boundaries and within grains increase creep resistance and rupture strength, having general characteristics of dispersion hardening.     

The strain induced martensite transformation in austenitic stainless steels: Part 2 – Effect of internal stresses on mechanical response

Abstract

In Part 1 of the present study attention was given to the influence of the deformation temperature on the structural aspects of the strain induced martensite transformation in austenitic stainless steels. In this part the mechanical response is examined, both during and after the martensite transformation, with particular attention to the development of internal stress. The influence of internal stresses on the bulk mechanical response concerns both the nature of the elastoplastic transition and the extent of kinematic hardening. The approach taken to assess the development of internal stresses is to use strain reversal (Bauschinger) tests and tensile experiments performed on a neutron diffractometer. In this way the evolution of stresses in partially transformed samples can be measured in the absence of any further martensite transformation.     

Effect of welding parameters on diffusion bonding of type 304 stainless steel–copper bimetal

Abstract

Stainless steel AISI type 304 and electrolytic cold rolled copper were joined by diffusion bonding at temperatures ranging from 650 to 950°C, for times from 5 to 45 min, and at pressures from 2 to 12 MPa. After bonding the microstructure of the interface was investigated, including the grain size, and shear and tensile strengths of the bonded specimens were determined. From the results, it was seen that the bond shear strength was dependent on interface grain boundary migration and on grain growth during the bonding process. In addition, attempts were made to find a relationship between grain size and shear strength in the bonding area. Taking into account the results of shear testing and microstructural observation, for a sound bond, optimum bonding conditions were obtained at temperatures of 800–850°C for 15–20 min at 4–6.5 MPa. The fracture behaviour of the diffusion bonded joint was investigated by means of shear and tensile testing under different bonding conditions. It was found that both shear and tensile strengths of the bonds were sensitive to the bonding conditions, and the intermetallic phases did not affect these parameters. Furthermore, the value of shear strength of the bond surface determined by shear testing was higher than the shear strength of the fracture surface determined by tensile testing.     

Effect of stabilising elements on formation of σ phase in experimental ferritic stainless steels containing 39%Cr

Abstract

The effect of additions of the stabilising elements aluminium, niobium, titanium, and zirconium on the formation of σ phase in the experimental ferritic stainless steel Fe–39Cr–2Mo–2Ni (wt-%) has been studied. It was found that additions of titanium and zirconium increased the overall rate of formation of σ phase, whereas additions of aluminium and niobium decreased the rate. Precipitation of σ phase can be significantly retarded by making an addition of from 1 to 3%Al. The morphology of the σ phase formed in these alloys is discussed. Time–temperature–transformation curves for some of the alloys are presented.

MST/1567     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Effect of primary α content on creep and creep crack growth behaviour of near α-Ti alloy

Abstract

The effect of primary α content on creep and creep crack growth behaviour of a near α-Ti alloy has been investigated at 600°C. The alloy was heat treated at different temperatures so as to obtain different volume fractions of equiaxed primary α in the range from 5 to 40%. Constant load creep tests were carried out at 600°C in the stress range 250–400 MPa until rupture of the specimens. Creep crack growth tests were carried out at 600°C and at an initial stress intensity level of 25 MPa m^1/2. Creep data reveal that minimum creep rate increases and time to rupture decreases with increase in primary α content indicating that higher primary α leads to creep weakening. On similar lines, maximum creep crack growth resistance is associated with the alloy with lowest primary α content (i.e. 5%). Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces in the interior of the specimen ahead of the crack tip.     

Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

Effect of carbon content on hydrogen occlusivity and embrittlement of ferrite–pearlite steels

Abstract

Experiments on a series of pure Fe–C alloys consisting of ferrite and pearlite only have shown that the ferrite/pearlite and pearlite/pearlite interfaces are effective hydrogen trapping sites. The ferrite/cementite interfaces within the pearlite colonies, however, have little effect on the hydrogen occlusivity. With an increase in carbon content, more ferrite/pearlite interfaces are created and these increase the hydrogen occlusivity. Although the ferrite/cementite lamella interface has little effect on the hydrogen occlusivity, it does appear that the lamellae interfere with the hydrogen diffusion path across the pearlite colonies. Thus, the higher-carbon alloys in the pearlitic condition have a lower apparent hydrogen diffusivity. Hydrogen has little effect on the tensile strength, but significantly reduces the ductility. After hydrogen charging, high-carbon alloys suffer a lower ductility loss. However, in terms of absolute values, the low-carbon specimens are always more ductile than the high-carbon alloys when saturated with hydrogen.

MST/433     

Influence of surface modification and long-term exposure on mechanical creep properties of γ-TiAl G4

Abstract

An investigation of the corrosion processes were performed for coated and uncoated γ-TiAl G4, an alloy designed to work in the temperature range 750 – 800°C, where oxidation and corrosion phenomena occur. An aluminising pack cementation treatment was used to improve the oxidation resistance of this γ-TiAl G4 alloy. Cyclic corrosion tests were performed at 800°C in air for up to 800 1-hour cycles with a Na₂SO₄/NaCl mixture. The influence of both aluminisation and the corrosion phenomena on the creep behaviour was investigated. The cyclic corrosion resistance of the coated γ- TiAl G4 was shown to be improved by aluminising. The pack cementation treatment had no detrimental effect on the creep behaviour. Moreover, neither is creep affected by the corrosion of coated specimens. As corroded uncoated specimen exhibited good creep behaviour, it can be concluded that this alloy is suitable, even without coating, for turbine applications in hot corrosion atmospheres at least up to 800°C.     

Effect of M7C3→M23C6 transformation on fracture behaviour of cast ferritic stainless steels

Abstract

The fracture behaviour of three 29 wt-%Cr ferritic steels, two containing zirconium and titanium respectively, has been investigated in the as cast condition and after annealing at 660°C for different times up to 2210 h. The fracture energy and the mode of fracture depend on both the morphology and the nature of the eutectic, which consists of carbides and ferrite. In the as cast condition, fracture is predominantly transgranular cleavage and it can be associated with the discontinuous morphology of the M₇C₃ carbides present in the eutectic as coarse particles surrounded by the eutectic ferrite. After prolonged heating, the ambient fracture energy decreases and the interdendritic mode of fracture is enhanced. This change in fracture mechanism is associated with transformation of the M₇C₃ to M₂₃ C₆ carbides. The M₂₃ C₆ carbides, unlike the coarse M₇C₃ carbides, form a continuous network within the eutectic mixture and constitute an easy path for crack propagation. The zirconium and titanium additions result in a more massive morphology of the carbides in the eutectic mixture and accelerate the M₇C₃ to M₂₃C₆ transformation during the heat treatments, enhancing the interdendritic mode of fracture both in the as cast and in the annealed condition.

MST/1734     

Synthesis and properties of metal matrix composite foams based on austenitic stainless steels –titanium carbonitrides

Metal matrix composite foams based on 316L stainless steel and reinforced with TiC0.7N0.3 were produced by the replication method using polyurethane sponge as a template. The rheological properties of the slurry appeared to be the key issue in the preparation of the composite foams. A homogeneous distribution of TiC0.7N0.3 particles throughout the 316L matrix and a good interaction between the 316L matrix and TiC0.7N0.3 reinforcement particles were obtained. Compression strength results showed that TiC0.7N0.3 particles acted as the real reinforcement medium. The values of the compressive yield strength and the elastic modulus of the metal matrix composite foams increased significantly with increasing TiC0.7N0.3 content when compared to the open cell 316L stainless steel foams.     

Development of structure and effect of processing parameters on Strength–structure relationships for two ferritic stainless steels

Abstract

The development of substructure as material passes through the roll gap has been examined for a ferritie stainless steel and it is shown that the final structure evolves close to the roll gap exit. Temperature variations as the material was rolled were monitored and the subsequent effect on substructure investigated. The variation of substructure with processing for two ferritic steels is discussed and the effect of this variation on room temperature properties is presented. It is shown that the strength–structure relationship is highly dependent upon retained martensite which degrades the tensile strength.

MST/380     

Effect of thermal aging on pitting corrosion resistance of 16Cr – 5Ni – 1Mo precipitation hardening stainless steel

Abstract

Specimens of precipitation hardening 16-5-1 stainless steel were solution treated at 1050°C for 1 h followed by aging at temperatures in the range 400 – 750°C for various holding times (1 – 16 h). After heat treatment, two types of corrosion test (accelerated and immersion testing) were conducted in 6% ferric chloride solution. The results showed that the pitting corrosion resistance was affected by austenite content, δ ferrite and precipitation of molybdenum and chromium carbides. Three critical temperature ranges were identified, which were related to the phases formed: (a) high corrosion rate at 475°C (δ ferrite and Mo₂ C); (b) low corrosion rate at 550 – 625°C (reversed austenite and Laves phase); (c) intermediate corrosion rate at 750°C (Cr₂₃ C₆ and TiC). The morphology of the pitting was dependent on the form of the δ ferrite and carbides.