Formation of ultrafine grained ferrite during thermomechanical treatment in microalloyed steel

Abstract

In the present work, the formation of ultrafine grained ferrite has been studied by applying suitable thermomechanical treatment. A high amount of deformation (～80%) at varying strain rates (0·01–10 s^?1) was applied in the temperature range of Ar₃ to Ac₃ followed by water quenching. This treatment resulted in a two-phase ferrite–martensite microstructure as compared to fully martensite structure after quenching without deformation. The formation of ultrafine ferrite (?3 μm) during deformation was favourable at a lower temperature and a slower strain rate. A maximum ～50% ferrite formed during deformation at 780°C with a strain rate of 0·01 s^?1. Experimental rolling with a high strain (～1·3) with finish rolling temperature just above Ar₃ (～750°C) resulted in fine ferrite–pearlite of ?3 μm, and the properties showed a high value of strength as compared to steels rolled in a conventional way. Dual phase microstructure (ferrite and martensite) was produced after partial austenisation to 780°C followed by quenching in water, and this resulted in an excellent combination of properties (high ultimate tensile strength, low yield strength/ultimate tensile strength, high elongation and high n values).     

Stepped heat treatment for austempering of high manganese alloyed ductile iron

Abstract

A stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron. The method is illustratedfor an iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, afirst step austempering temperature of 400°C for 120 min, and a second step austempering temperature of 285°C. The change in the microstructure and phase characteristics with time during the second austempering step are described. Related changes in the mechanical properties compared with a single austempering treatment at 400°C are an increase in the ultimate tensile strength from 770 to 970 MN m^?2, an increase in elongation from 2·5 to 7·5%, and an increase in the unnotched Charpy impact energy from 40 to 150 J.

MST/3119     

Computer modelling of phase diagrams

Abstract

An investigation has been made of the tensile behaviour between 20 and 600°C of two ultrahigh boron steels (Fe–2·2B and Fe–4·9B), consolidated by hot isostatic pressing at temperatures ranging from 700 to 1100°C. Tensile tests showed plastic deformation only in the Fe–2·2B alloy. A decrease in yield and ultimate tensile stresses occurred when the consolidation temperature was increased. This was accompanied by an increase in the elongation to failure. This alloy satisfies the Hall–Petch relation for all testing temperatures. The slope of the yield stress versus d^?1/2 curve (d is grain size) decreases as the temperature increases, indicating that the mechanism controlling plastic deformation becomes independent of grain size at high testing temperatures. The fracture mode observed was brittle at room temperature and ductile, shown by the presence of dimples, at temperatures above 400°C.

MST/2050     

Influence of process parameters on superplasticity of friction stir processed nugget in high strength Al – Cu – Li alloy

Abstract

A parametric study was carried out to evaluate the influence of friction stir processing (FSP) parameters (tool rotation speed and feed rate) on the superplasticity of the weld nugget. Dynamically recrystallised AA 2095 thin sheets with a fine grain size of 2 μm were welded using four feed rates and three rotational speeds. High temperature tensile testing was employed to understand the significance of the FSP parameters and to optimise the parameters for maximum elongation. The tool rotation speed was found to be the most decisive parameter for controlling superplastic behaviour. A strain rate sensitivity of 0·68 was measured for the highest rotational speed at the optimum superplastic forming (SPF) temperature of 495°C. A maximum percentage 'elongation to failure' of 550% was achieved for the sheets subjected to FSP at 1000 rev min^?1 and 4·2 mm s^?1, compared with 475% obtained for the base metal at the optimum SPF temperature and strain rate of 495°C and 10^?3s^?1, respectively.     

Influence of strain rate on production of deformation induced ferrite and hot ductility of steels

Abstract

Compression testing was used to explore the influence of strain rate on the formation of deformation induced ferrite. Samples of a 0·4%C–1·4%Mn plain C–Mn steel were heated to 1225°C, cooled to test temperatures in the range 1100–610°C, and then given a true strain of 0·6, at strain rates of3 × 10^?2, 3 × 10^?3, and 3 × 10^?4 S^?1. At the lowest strain rate it wasfound that the strain to peak stress decreased with decreasing temperature in the range 750–610°C. This behaviour is related to the formation of thin films of the softer deformation induced ferrite at the γ grain boundaries at the higher temperatures, and spheroidisation at the lower temperatures. More normal stress–strain curves were observed at the higher strain rates, as raising the strain rate prevents the formation of deformation induced ferrite and delays spheroidisation. The strain rate was also found to have an important influence on the extent of recovery in the deformation induced ferrite; the lowest strain rate enabling full recovery and or recrystallisation to occur, thus keeping the film soft. This behaviour is shown to account for the poor hot tensile ductility at the lowest strain rates. Raising the strain rate in this temperature range improves the ductility because work hardening takes place, raising the strength of the ferrite closer to that of the y, thus preventing strain concentration from occurring.

MST/1934     

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Effect of process parameters on structure and macrosegregation upon direct chill casting of Mg–Nd–Zn–Zr alloy

Abstract

Direct chill (DC) semicontinuous casting process has been successfully used to produce sound Mg–3·0Nd–0·4Zn–0·4Zr (NZ30K) billets. The influence of process parameters such as casting speed, casting temperature on the microstructure and macrosegregation was studied. The results show that the casting speed affects the macrosegregation greatly while it has a slight influence on the grain size of the billet; the casting temperature has a slight influence on macrosegregation of the billet while the grain size of the billet increases as the casting temperature increases. The optimal process parameters have been experimentally determined as follows: casting temperature 700°C and casting speed 90 mm min^?1. The ultimate tensile strength, yield strength and elongation of billets cast at the optimal casting parameters are 196 MPa, 125 MPa and 16·5% respectively.     

Effect of nitrogen content on dynamic strain ageing behaviour of type 316LN austenitic stainless steel during tensile deformation

Abstract

The effect of nitrogen content on the dynamic strain ageing (DSA) behaviour of type 316LN austenitic stainless steel has been studied. The nitrogen content was varied from 0·07 to 0·22 wt-%. The tensile tests were carried out over a temperature range of 300–1123 K and at three strain rates in the range 3×10^?3–3×10^?5 s^?1. Serration was observed in the load elongation curves in the intermediate test temperature range and has been considered due to DSA phenomenon. The critical strain to onset of serrated flow increased with increase in nitrogen content and strain rate. The temperature for onset of DSA and the temperature of disappearance of DSA were found to increase with the increase in nitrogen content. The variations in tensile strength and work hardening rate of the steel with temperature exhibit peak values in the intermediate temperature range and have been attributed due to DSA phenomenon. The activation energy for DSA, estimated based on the temperature and strain rate dependences of the strain to onset of serrated flow, was found to increase from 111 to 218 kJ mol^?1 with the increase in nitrogen content from 0·07 to 0·22 wt-% and the increase has been attributed to the possible enhanced interaction of the DSA causing interstitial nitrogen with substitutional chromium.     

Dynamic recrystallisation and dynamic precipitation in AA6061 aluminium alloy during hot deformation

Abstract

Deformation behaviour of AA6061 alloy was investigated using uniaxial compression tests at temperatures from 400 to 500°C and strain rates from 0·01 to 1 s^?1. Stress increases to a peak value, then decreases monotonically until reaching a steady state. The dependence of stress on temperature and strain rate was fitted to a sinh-Arrhenius equation and characterised by the Zener–Hollomon parameter with apparent activation energy of 208·3 kJ mol^?1. Grain orientation spread analysis by electron backscattered diffraction indicated dynamic recovery and geometrical dynamic recrystallisation during hot compression. Deformation at a faster strain rate at a given temperature led to finer subgrains, resulting in higher strength. Dynamic precipitation took place concurrently and was strongly dependent on temperature. Precipitation of Q phase was found in the sample deformed at 400°C but none at 500°C. A larger volume fraction of precipitates was observed when samples were compressed at 400°C than at 500°C.     

Effect of strain rate on tensile deformation and fracture behaviour of 18%Ni 300 maraging steel sheet

Abstract

Deformation and fracture under uniaxial tensile loading at room temperature were investigated for 18%Ni 300 maraging steel sheet in the strain rate range 1·67×10^?5 to 1·67× 10^?1s^?1. The steel showed an increase in flow stress with strain rate and the increase in yield strength (YS) was more pronounced compared with the tensile strength (TS), resulting in a corresponding decrease of TS/YS ratio. Both the level of deformation and the deformation zone were also reduced by the increasing strain rate. Fractographic analysis indicated that the increasing strain rate induced, to some extent, plane strain constraint in the sheet resulting in increasing fracture angle, decreasing ductility/fracture strain, and increasing dimple size. With increasing strain rate the work hardening rate dσ/d? and strain hardening coefficient (n value) of the steel also decreased; hence, correlations were found between dσ/d?, TS/YS ratio, and n value. The decrease of these three parameters caused strain localisation as confirmed by the presence of intergranular dimples and intergranular shear. Also, the dimple density decreased as the strain rate was increased.

MST/729     

Effect of consolidation temperature on mechanical properties of rapidly solidified Al–7Mg–1 Zr alloy

Abstract

The mechanical properties achieved via the extrusion of non-degassed billets prepared from an inert gas atomised powder of nominal composition Al–7Mg–lZr are reported. The alloy was extruded over the temperature range 350–550°C and the tensile mechanical properties and plane strain fracture toughness were evaluated. It was found that the yield strength remained fairly constant over the entire temperature range, with only a small decrease in strength observed at the highest extrusion temperature. The strength could be related to microstructure using standard models for solid solution, dispersoid, and substructural strengthening mechanisms, and the last was found to make the greatest contribution. The sensitivity of strength to subgrain size was found to be nearly three times higher than that for pure Al. The optimum combination of strength and fracture toughness was obtained for extrusion at 500°C (yield strength 400 MN m^?2; T–L K_Iv 21 MN m^?3; elongation 20%). The poor values of K_lv obtained at other temperatures were attributed to coarse dispersoids (highest extrusion temperature), undeformed powder particles (lowest extrusion temperature), and inhomogeneous dispersoid distributions (intermediate temperatures). It is concluded that extrusion process control plays an important role in determining the mechanical properties of consolidated rapidly solidified powders. Considering the excellent ductility and toughness obtained, vacuum degassing before extrusion may not be essential in the processing of inert gas atomised powders of a non heat treatable composition.

MST/1721     

Effects of pearlite formation on mechanical properties of austempered ductile iron

Abstract

Mechanical property measurements are described for a ductile iron alloyed with Mn, Mo, and Cu in the fully austempered condition, andfor irons with various amounts of pearlite introduced by isothermal transformation at 550°C after austenitising at 920°C for 120 min and before austempering at 370°C for 60 min. The ultimate tensile strength, 0·2% proof strength, elongation, impact energy, and hardness all decrease as the amount of pearlite in the structure increases. A smaller amount of pearlite can be tolerated in the alloyed iron compared with an unalloyed iron before itfails to satisfy the standard.

MST/2041     

Post-forming tensile properties of superplastically bulge formed high strength α/β Ti–Al–Mo–Sn–Si alloy (IMI 550)

Abstract

The effect of biaxial superplastic deformation at 900°C on ambient temperature tensile properties and texture of high strength α/β titanium alloy sheet of nominal composition Ti–4Al–4Mo–2Sn–0·5Si (IMI 550) has been examined. Superplastic straining led to significant decreases in both proof stress and tensile strength values. Heat cycling studies on as received sheet material showed that the decreases in strength were in part due to the influence of temperature, but this had little effect on elongation. The further losses in strength after superplastic forming were attributable to grain growth. The decreases in elongation after superplastic deformation were believed to be due mainly to changes of tensile specimen geometry, while the more isotropic tensile behaviour which was observed was due to the gradual removal of the relatively low level of texture in the as received material. Application of the standard heat treatment to the as received IMI 550 sheet material led to increases in proof stress and tensile strength values of ~70 and ~170 MN m^?2, respectively, and to a slight decrease in elongation. Heat treatment of heat cycled and superplastically bulge formed specimens increased the proof stress and tensile strength values almost to the levels attained in the as received material.

MST/684     

Influence of deformation induced ferrite,grain boundary sliding,and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence of C on hot ductility in the temperature range 600–1000°C has been examined for three C contents (0·1, 0·4, and 0·75 wt-%). Using a strain rate of 3 × 10^?3 s^?1, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 × 10^?2 and 3 × 10^?4 s^?1 were examined. At the strain rate of 3 × 10^?3 s^?1, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the γ–α transformation. Thin films of the softer deformation induced ferrite formed around the γ grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae₃) and dynamic recrystallisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the 0·1%C steel corresponded to the presence of a large volume fraction of ferrite, this being the more ductile phase. For the 0·4%C steel decreasing the strain rate to 3 × 10^?4 s^?1 resulted in a very wide trough – extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding in the γ. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation induced ferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae₃, resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·1%C.

MST/1809     

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     

Microstructure evolution of ZK40 magnesium alloy during high strain rate compression deformation at elevated temperatures

Abstract

Microstructure evolution of the homogenised ZK40 magnesium alloy was investigated during compression in the temperature range of 250–400°C and at the strain rate range of 0·01–50 s^?1. At a higher strain rate (?10 s^?1), dynamic recrystallisation developed extensively at grain boundaries and twins, resulting in a more homogeneous microstructure than the other conditions. The hot deformation characteristics of ZK40 exhibited an abnormal relationship with the strain rate, i.e., the hot workability increased with increasing the strain rate. However, the dynamic recrystallisation grain size was almost the same with increasing the temperature at the strain rate of 10 s^?1, while it increased obviously at the strain rates of 20 and 50 s^?1. Therefore, hot deformation at the strain rate of 10 s^?1 and temperature range of 250–400°C was desirable and feasible for the ZK40 alloy.     

Development of new low nickel,cobalt free maraging steel

Abstract

A new low Ni, Co free maraging steel containing 12Ni–3·2Cr–5·1Mo–ITi (wt-%) has been developed. The optimum heat treatment was found to be solution treatment at 1098 K for 60 min followed by air cooling then aging at 753 K for 180 min. Detailed optical and transmission electron microscopy revealed that the solution treated steel transformed to lath martensite. The optimised steel achieved an ultimate tensile strength of 1700 MN m^?2 and a yield strength of 1660 MN m^?2. The toughness measured in terms of impact energy was found to be 38 J. Fractographic analysis carried out in the scanning electron microscope showed predominantly dimpled structures indicating ductile failure. The transformation temperatures for this new steel were established using dilatometry.

MST/1398     

Influence of test direction on hot ductility of austenite

Abstract

The influence of test direction on the hot ductility of a hot worked high S steel (0·15%S) has been examined over the temperature range 700–1100°C and for strain rates 3·3 × 10^?4 and 1·3 %times; 10^?2 s^?1. As with room temperature tensile testing, ductility was lowest in the short transverse direction and greatest in the longitudinal direction. Ductility troughs were observed for all directions at the lower strain rate. Increasing the strain rate improved hot ductility and removed the trough for samples tested in the longitudinal direction. Similar directional behaviour was observed for plate steel with standard S levels (≤0·01%), but the differences in hot ductility with direction were much reduced. Fracture was transgranular dimpled rupture when ductility was good, but intergranular for poor ductility. Intergranular failure occurred when the steels were austenitic. The elongated MnS inclusions were found to be situated in the γ-boundaries and it is believed that intergranular failure occurred by the inclusions restraining grain boundary movement and allowing voiding and decohesion at the inclusion/boundary interface, thus encouraging grain boundary sliding. Increasing the strain rate improved ductility by reducing the amount of grain boundary sliding and increasing the grain boundary migration rate making it difficult for cracks to link up.

MST/776     

Strength,toughness, and stiffness of wrought and directly sintered T6 high-speed steel at 20–600°C

Abstract

The mechanical properties of directly sintered T6 high-speed steel in the temperature range 20–600°C were generally comparable to those of concurrently heat-treated wrought material of similar composition. For the hardness range 860–940 HV30 macroscopic ductility was detected at 200°C and 450°C in the wrought and sintered materials, respectively; failure strains, however, did not exceed 2%. The value of Young's modulus dropped from ~240 to ~120 GNm^?2 as the temperature was raised to 600°C, yield strength dropped from 2·2 to 1·0 GNm^?2, but the fracture strengths showed a maximum, ~2·1 GNm^?2 at ~400°C for the wrought steel and ~1·4 GNm^?2 at ~450°C for the sintered steel. Microcracking preceded yielding and/or failure and was mainly through carbides, which were generally below the critical size to cause catastrophic fracture. The second stage of the failure process involved the linking through the matrix of such microcracks until conditions for fast fracture were satisfied (stage three). A quantitative model for carbide cracking in high-speed steels is absent as is the correlation of fracture strength with fracture toughness via the critical defect size, since, for example, the failure originating zones in wrought samples identified by scanning electron microscopy were generally larger than those predicted by linear elastic fracture mechanics (LEFM). It is suggested that there may be some analogies between failure in monotonic loading of high-speed steels and of ceramics with small defects; the behaviour in fatigue of short cracks in alloys and microscopic crack growth in delayed fracture of ceramics where LEFM analyses developed as a result of studying artificial long cracks appear not to hold.

MST/606     

Solidification structures and mechanical properties of Zn–27Al alloy cast in metal moulds

Abstract

Rectangular and round cross-section test bars of Zn–Al alloy containing (wt-%) 27·67%Al; 2·30%Cu; 0·13%Mg; 0·04%Fe; <0·0025%Pb, Cd, and Sn; balance Zn were cast in metal moulds. The variables studied were: bar cross-section 3–25 mm thickness and 14·5–28 mm dia., pouring temperature 550–700°C, feeder location at bar shoulders or over full gauge length, and cast iron mould preheating temperature 100 or 200°C. The results show that the above variables can lead to variations of 0·14–1·72% bar porosity, 18–0·5% elongation, 428–305 MN m^?2 tensile strength, and 108–128 Brinell hardness. Metallographic examination leads to the conclusion that these properties are influenced by the volume, distribution, and morphology of shrinkage porosity and of eutectic constituent. The observed variations in the microstructure and their effects on the mechanical properties are related to the cooling and feeding conditions of the bars.

MST/549