Improved mechanical properties of ultrahigh strength 0·4C–Cr–Mo–Ni steel through modification of sulphide inclusion shape and microstructural control

Abstract

A new approach is suggested whereby the mechanical properties of commercially available 0·4C–Cr–Mo–Ni ultrahigh strength steel can be significantly improved whether the testing orientation is longitudinal or transverse. The new approach consists of combining the decreased hot rolling reduction treatment with a short time isothermal transformation treatment. This approach, compared with conventional heat treatment, resulted in significant improvements of the plane strain fracture toughness K_Ic at increased strength and of the Charpy V-notch impact energy, for either testing orientation. Steel treated using the new approach also had an improved combination of mechanical properties compared with steels treated using high austenitisation and controlled rolling, which are possible approaches to improving the values of K_Ic or other mechanical properties of steels. The favourable effect on the mechanical properties is discussed in terms of metallography, X-ray measurements, and fractography.

MST/984     

Structure,anisotropy, and properties of hot rolled AA 5083 alloy

Abstract

The changes in structure and substructure occurring during homogenisation and hot rolling of an Al–5Mg alloy (AA 5083) have been investigated. It is shown that a homogenisation treatment is beneficial and that the resulting structure can be related to processing parameters. The results suggest that the substructure morphology is dependent upon the total strain, but this has not been quantified. The development of texture was also studied and it is shown to be almost invariant with temperature, but strongly strain dependent. The anisotropy so produced yielded plastic strain ratios that were found to be strongly dependent upon the rotated copper texture intensity. The deep drawing behaviour of the hot rolled sheet was investigated by employing cupping tests and it is shown that a relationship exists between the earing value and the subgrain size.

MST/I086     

Effect of homogenisation treatment on segregation of silicon in ferritic ductile irons: a colour metallographic study

Abstract

Microsegregation strongly affects the properties of ferritic ductile irons. A homogenisation treatment was attempted to decrease such segregation. The severity of segregation can be determined with microprobe analysis, but this technique cannot reveal the morphology of segregation on a larger scale. The present investigation was undertaken to explore the use of colour metallography for revealing the severity of segregation both qualitatively and quantitatively during the homogenisation treatment. The modification of the morphology of segregation of silicon during homogenisation treatment for different times was revealed clearly by colour metallographic techniques. The proportion of interdendritic areas to the total matrix area was quantified. A semi-quantitative relationship between the difference in the sequence of colour and the degree of segregation of silicon in ferritic ductile iron was developed with the use of a combination of a colour metallographic technique and microprobe analysis.     

Intermetallic and dispersoid structures in aa3104 Aluminum alloy during two-step homogenisation

During homogenisation of the AA3104 cast ingot, a phase transformation of intermetallic particles from β-Al₆(Fe,Mn) orthorhombic phase to harder α-Al_x(Fe,Mn)₃Si₂ cubic phase occurs. The large constituent intermetallic particles, regardless of phase, assist in the recrystallisation nucleation process through particle stimulated nucleation (PSN). Ultimately, this helps to refine grain size. The sub-micron dispersoids act to impede grain boundary migration through a Zener drag mechanism. For this reason, the dispersoids that form during homogenisation are critical in the recrystallisation kinetics during subsequent rolling, with smaller dispersoids being better suited to reverse rolling mills. This work simulates an industrial two-step homogenisation practice with variations in the peak temperature of the first step between 560 °C and 580 °C. The effect of this temperature variation on the intermetallic particle-phase evolution is investigated. The aim is to identify the ideal intermetallic phase balance and the dispersoid structure that are best suited for hot rolling on a single stand reversing mill, in order to minimise recrystallisation during rolling through maximising Zener drag and maintaining galling resistance. The results indicate a trend where an increase in homogenisation temperature from 560 °C to 580 °C yields, firstly, an increase in the volume fraction of the α-phase particles to greater than 50% of the total volume fraction at both the edge and the center of the ingot and, secondly, it yields an increased dispersoid size. Thus, in the context of a reverse rolling operation, a lower temperature homogenisation practice produces a near-ideal combination of intermetallic particle-phase distribution, as well as dispersoid size, which is critical for Zener drag and the minimization of recrystallisation during the hot rolling processes.

Graphical abstract

SEM BEI images and corresponding EDS maps, highlighting the variation in intermetallic particle phase balance, size and morphology after homogenisation at different temperatures. With a focus on the exaggerated differences seen between material the center of and at the edge of a DC cast ingot of AA3104 Aluminum alloy.

     

Control of microstructure and earing behaviour in aluminium alloy AA 3004 hot bands

Abstract

Control of earing behaviour at the hot band stage is a critical requirement for successful manufacture of aluminium alloy sheet for beverage cans. The present study has combined production scale experiments with laboratory examinations to investigate the effect of various material and process parameters on microstructure, texture, and earing of the resulting products. It is shown that optimisation of the product is strongly dependent on (i) iron content of the alloy, (ii) ingot homogenisation temperature, (iii) finish hot rolling temperature, and (iv) heating rate during hot band annealing. Earing level after annealing is shown to depend on the balance between cube (+ Goss) texture intensity and the volume of material having almost randomly spread orientations. Pronounced 0/90° earing tendency is usually associated with coarse and elongated grain structures. A model is shown which represents the microstructure–texture evolution as a competition between cube/Goss grains, which nucleate systematically within transition bands, and randomly oriented grains, which nucleate in the vicinity of coarse second phase particles.

MST/1032     

Microstructural control by secondary processing of strip cast low carbon steel

Abstract

The secondary processing of low carbon steel strip produced by twin roll casting was investigated to examine its effect on microstructural development and mechanical properties. The as cast microstructure is predominantly acicular ferrite with regions of bainitepearlite and polygonal ferrite. Deformation at temperatures below Ar1 produces a heterogeneous microstructure with regions of moderately deformed acicular ferrite adjacent to highly deformed regions containing shear bands. Cold rolled and warm rolled steels show similar behaviour to conventional hot band in that dynamic recovery during warm rolling results in sluggish recrystallisation and produces a coarse final grain size. However, the initial as cast microstructure recrystallises at a slower rate than conventional hot band and produces a weaker recrystallisation texture. This can be attributed to the heterogeneous microstructure of the as cast strip such that, after rolling, nucleation occurs within shear bands and more ill defined sites, which results in nucleation of randomly oriented grains thereby producing a weak final texture. It was found that austenitising the as cast strip followed by rolling in the vicinity of Ar3 produces a uniform distribution of equiaxed, ultrafine ferrite UFF grains throughout the thickness of the strip. The production of UFF by twin roll casting and subsequent rolling represents a simple processing route for the production of fine grained low carbon sheet steel products.     

马氏体时效钢带状组织产生的原因及消除方法

马氏体时效钢具有超高强度和良好的综合性能。但是,该合金在锻造成形过程中存在大量带状组织,这种组织严重影响了材料的使用性能,如使材料各向异性,塑性降低,冲击韧性及断面收缩率下降等。本文重点探讨了带状组织产生的内因,并通过热处理工艺试验探索了消除带状组织的方法,试验研究表明,合金经过1050℃×2h＋820℃×1h热处理后,材料的带状组织基本碎化或消除,组织和成分细小均匀,有效地改善了材料的力学性能。     

Segregation and diffusion characterisation in two-stage homogenisation of conventional superalloy

Abstract

The segregation behaviour of φ 406 mm IN718 ingot was investigated, and the experimental results indicated that Nb was the most segregated element. Two-stage homogenisation treatment was employed to relieve the extensive microsegregation in the as cast ingot. Historically, it was difficult to characterise the effects of the second stage homogenisation because of the uncertainty in locating the diffusion areas. To solve this problem, a 900°C/1 h treatment (‘Tag’ method) was conducted after homogenisation, which could form δ phase (Nb enriched) to trace the diffusion of Nb. The results revealed that the average composition of δ phase area, quantity of δ phase precipitates and sizes of δ phase precipitates change with the proceeding of homogenisation. Eventually, homogeneous distribution of elements could be achieved by means of two-stage homogenisation treatment (1140°C/60 h+1190°C/30 h).     

Fabrication of Al–Cu laminated composites by diffusion rolling procedure

Abstract

A diffusion rolling procedure was employed for the fabrication of Al–Cu laminated composites; the microstructure and mechanical properties of the interface were investigated. With diffusion bonding initially, intermetallic compounds (IMCs) occurred at the Al/Cu interface. After plastic deformation by rolling the laminated composites, the interface strip of IMCs broke and became discontinuous equiaxed particulates. Compared with roll bonding with heat treatment and diffusion bonding, the shear tensile strength of two-stage processed Al/Cu interface reached a maximum value equivalent to 90% of that of Al. Therefore, it is concluded that the diffusion rolling procedure yielded the highest strength of Al–Cu laminated composites.     

Characteristics of TiC dendrites in as solidified Ti–Al–C alloys

Abstract

Results are reported on the dendrite secondary arm spacing of a series of as cast Ti–C and Ti–Al–C alloys in the composition range up to 10 at.-%C and 15 at.-%Al. The presence of Al leads to a significant decrease in the dendrite spacing, an effect of potential interest for improving mechanical properties. The structural refinement is attributed mainly to the slower diffusion of Al as compared with carbon, in the solute partitioning required for coarsening of dendrite arms.     

Particle break-up during heat treatment of 3000 series aluminium alloys

Abstract

In wrought aluminium alloys, the fragmentation of coarse, iron bearing intermetallic particles by hot rolling is an important development in industrial processing. Here a model 3000 series alloy is used to show that fragmentation can occur prior to hot rolling, during the homogenisation heat treatment. Some fragmented particles display a curved morphology of break-up that results from matrix wetting of two phase (or 'duplex') interfaces in Al₆(Fe,Mn) particles partially transformed to an α-Al–(Fe,Mn)–Si phase. In contrast, samples rapidly heated to temperature in a fluidised bath show an angular break-up indicative of tensile stresses induced by thermal expansion mismatch between the intermetallic particles and aluminium matrix. Although this break-up should not be industrially significant, the transformation induced break-up by wetting may be. More generally, internal boundaries resulting from the transformation to α-Al–(Fe,Mn)–Si phase may be mechanically weak fracture initiation points during hot rolling.     

Solute segregation effect on grain boundary migration and Hall–Petch relationship in CrMnFeCoNi high-entropy alloy

Homogenisation has been used to eliminate solute segregation of CrMnFeCoNi high-entropy alloys (HEAs) before thermomechanical processing. To overcome the deficiencies caused by high-temperature homogenisation, we cancelled the homogenisation and studied the solute segregation feature and its effect on the grain growth and the mechanical property. The HEA forms equiaxed grains with the solute segregation in the range of tens of micrometres after cold rolling and recrystallisation. The grain growth in recrystallisation still abides by the classical grain growth kinetics, but with a higher power index of 3.33 and activation energy of 392.4?kJ?mol^?1 than those of the homogenised HEA, indicating a solute-drag effect. The relationship between the yield stress and grain size follows the Hall–Petch dependence with a higher intrinsic strength.     

Influence of thermomechanical processing on superplastic forming of Mg–Al alloys

Abstract

The aim of this paper is to study the influence of the initial microstructure of several Mg–Al alloys on their superplastic formability and on their post-forming microstructure and mechanical properties. Various thermomechanical processing routes, such as annealing, conventional rolling, severe rolling and cross rolling, were used in order to fabricate AZ31 and AZ61 alloys with different grain sizes. These materials were then blow formed into a hat shaped die. It was found that the processing route has only a small effect in the formability of Mg–Al alloys or on the post-forming microstructures and properties due to rapid dynamic grain growth taking place at the forming temperatures. Nevertheless, good formability is achieved as a result of the simultaneous operation of grain boundary sliding and crystallographic slip during forming.     

Effect of process variables on tensile properties of ingot processed versus strip cast iron aluminides

Abstract

The effects of cold rolling and subsequent heat treatment are reported for ingot cast-hot rolled and strip cast iron aluminides (23–29 at.-%Al). Partially recrystallised microstructures obtained by hot rolling at 800°C gave higher strength and elongation to fracture than hot rolling at 1000° C. Elongation increased and proof strength decreased in proceeding from DO₃ to mixed B2 + DO₃ and to B2 order as a result of heat treatment following cold rolling. Variability in the properties of strip cast material was associated with casting defects such as laps, but the best properties were comparable with those of ingot processed material. The detrimental effects of tensile specimen preparation by spark erosion are discussed.     

Effect of modified austemper on tensile properties of 0·52%C steel

Abstract

The effect of a modified austemper on the tensile properties of 0·52%C steel has been studiedfor the purpose of developing the mechanical properties of upper bainitic steel. The modified austempering treatment involved intercritical annealing at 1018 K in the two phase region offerrite (α) and austenite (γ) followed by austempering at 673 K and subsequent water cooling. The results have been compared with those obtained from conventionally austempered steel, and quenched and tempered steel with a similar ultimate tensile stress. The modified austempered steel consisted of a mixed structure of upper bainite and 10 vol.-% ferrite in which ferrite appeared as layers along the rolling direction. The modified austempering treatment wasfound to significantly increase the product of ultimate tensile stress and total elongation, and also the notch tensile stress at 193 K. Conventional austenitising at 1173 K followed by subcritical annealing at 998 K in the two phase region of ex and y, and then austempering at 673 K and subsequent water cooling produced the same mixed structure of upper bainite and 10 vol.-% ferrite. However, this treatment yielded inferior mechanical properties to those obtained with the modified austempering treatment, independent of the test temperature. The results are described and discussed.

MST/3102     

Phase evolution in mechanical alloying and spark plasma sintering of AlxCoCrCuFeNi HEAs

ABSTRACT

Al_xCoCrCuFeNi high-entropy alloys were synthesised through mechanical alloying and spark plasma sintering. Different alloys were produced by varying the aluminium content (x?=?0.5, 1.5, 2.5 and 4). The influences of the milling duration on the evolution of microstructure, constituent phases and morphology were studied. Increasing milling time resulted in grain refinement and higher solid solution homogenisation characterised by a high internal strain. As a consequence of aluminium addition, the microstructure of materials evolved from face centered cubic (FCC) and body centered cubic (BCC) phases to FCC, BCC and ordered BCC phases. Both mechanical alloying and SPS conditions as well as aluminium content led to grain refinement and variations of mechanical properties. In particular, hardness increased with increasing aluminium content. The aluminium percentage and the evolution of consequent phases are responsible for the microstructural stability at high temperatures. In addition, with Al content increase, the further synergy of strength and ductility along with a more pronounced strain hardening was obtained.     

Transient liquid phase bonding of magnesium alloy (Mg – 3Al – 1Zn) using copper interlayer

Abstract

The mechanism, microstructure and mechanical properties of transient liquid phase (TLP) bonded magnesium alloy (Mg – 3Al – 1Zn) joints using copper interlayers in an argon atmosphere have been investigated. The formation process of the TLP joint comprises a number of stages: plastic deformation and solid state diffusion, dissolution of the interlayer and base metal, isothermal solidification and homogenisation. The composition profiles and microstructures of the joint depend on the bonding time at a temperature of 530°C. With an increase in bonding time from 10 to 60 min, the concentration of copper and the amount of CuMg₂ compound in the joint decrease. For longer bonding times, the most pronounced features of the joint are composition homogenisation, grain coarsening and elimination of the bond line within the joint centre. The presence of brittle CuMg₂ and grain coarsening of the joint are the main reasons for impairing the joint shear strength. A joint shear strength of 70.2 MPa, which is 85.2% of the base metal strength (82.4 MPa), can be achieved by bonding at 530°C for 30 min.     

Nanoscale characterisation of rolling contact wear surface of pearlitic steel

Abstract

Nanoscale characterisation of a rolling–sliding wear surface layer of pearlitic steel was performed with transmission electron microscopy and atom probe tomography to reveal microstructural changes in the pearlite structure. Plastically deformed fine pearlitic lamellae with interlamellar spacing of ～10 nm were observed just beneath the contact surface after the rolling–sliding wear test, where the hardness of the surface reached >800 HV, twice the initial bulk hardness of 400 HV. Lamellar cementite was slightly decomposed, but most lamellar cementite was retained as thinned lamellae in the deformed pearlitic structure. The large increment in hardness was mostly explained by the reduction in interlamellar spacing. The formation mechanism of the microstructure of the worn surface was compared with that of the white etching layer on the pearlitic rail surface.     

Array sensor spacing for optimal time‐delay estimation

Abstract

The effects of sensor spacing on the performance of time‐delay estimation are considered. For narrow‐band signals the array aperture and sensor spacing seriously change the estimation result. The post beamforming signal‐to‐noise ratio is also important for choosing the sensor spacing. The conventional half wavelength spacing is shown to be the only one of the many choices towards achieving a similar estimation performance.     

Prevention of surface hot shortness,development of banded structure,and mechanical properties of hot rolled Cu-bearing steel

The development of microstructural banding and surface hot shortness during hot rolling in a 1.4 wt% Cu-bearing steel was studied. Different hot rolled states were produced by cross rolling, air cooling, and furnace cooling to investigate the effect of initial microstructure on the mechanical properties. It was revealed that to insure the hot workability of Cu-bearing steel against liquid metal embrittlement and prevention of the failure, a good practice is conducting hot working operations at temperatures below the melting point of copper to suppress the formation of liquid Cu-enriched phase that penetrates into grain boundaries. Cross rolling was found to be a promising approach to decrease the anisotropy of the rolled sheets resulted from the presence of the banded structure. Moreover, air-cooling yielded maximum strength due to its resultant fine and complex microstructure. These results can find application in processing and optimization of mechanical properties of steel sheets.