Jer-Ren Yang

Mechanical stabilisation of austenite

Abstract A theory has been developed for the mechanical stabilisation of plastically deformed austenite by balancing the force which drives the transformation interface against the resistance from dislocation debris in the austenite. The work has been used to explain why very large strains are required to mechanically stabilise certain stainless steels, and also to interpret the subunit mechanism of bainite growth.

Characterisation of severely deformed austenitic stainless steel wire

Abstract The microstructure of 8 μm diameter wire produced by the severe deformation of 316L austenitic stainless steel has been examined using TEM and X-ray diffraction. The deformation imparted amounts to a true strain of 6·3. Data from previous studies on strain induced transformation of this steel have been combined with new results to show that true strains >2 are required in order to observe mechanical stabilisation, i.e. the cessation of martensitic transformation when the martensite/austenite interfaces are unable to propagate through the dislocation debris created in the austenite.

Acicular ferrite transformation in alloy-steel weld metals

In this paper, the morphology of acicular ferrite in alloy-steel weld metals has been investigated. The effect of the grain size of prior austenite on acicular ferrite transformation has also been studied. It is found that acicular ferrite can form in reheated weld metals when the austenite grain size is relatively large. On the other hand, classical sheaf-like bainite will form at the same temperature if the austenite grain size is kept small. Further results strongly suggest that acicular ferrite is in fact intragranular bainite rather than intragranular Widmanstätten ferrite.

Effects of chemical composition, rolling and cooling conditions on the amount of martensite/austenite (M/A) constituent formation in low carbon bainitic steels

Abstract To understand the controlled-rolled granular bainite structure, a series of high-strength low-alloy steels was specially designed and investigated. The effects of the chemical composition, the finish-rolling temperature, and the cooling rate on the amount of martensite/austenite (M/A) constituent formation in the as-rolled alloy steels were evaluated. It was found that for steels containing the same addition of Nb (0.045 wt.%), the granular bainite transformation was retarded with an increased carbon content ranging from 0.021 to 0.056 wt.%. As the carbon content was raised, more niobium combined with carbon to form carbide, and the austenite therefore became niobium-depleted. This effect significantly influenced the granular bainite transformation for the steels studied. It is also shown that the effects of Mn and Mo on the formation of second phases are different from that of carbon. These alloying elements tend to promote the M/A constituent and depress pearlite formation. As to the effect of rolling temperature, it is shown that as the finish-rolling temperature is lowered, the larger strain accumulated in austenite enhances polygonal ferrite formation along the previous austenite grain boundaries, and reduces the amount of M/A constituent. Furthermore, based on a thermodynamic analysis, it is indicated that the granular bainite transformation terminates as the carbon content of austenite reaches the TO curve. The critical carbon content increases with the decrease of the granular bainite transformation temperature. It was also found that the amount of M/A constituent formation decreases with the increase in cooling rate. This result is not consistent with that reported by Shiga et al., Tetsu-to-Hagane, 68 (1982) A227 and Bufalini et al., Accelerated cooling of steel,

The effects of rolling processes on the microstructure and mechanical properties of ultralow carbon bainitic steels

Abstract In this work, experimental ultralow carbon bainitic steels were prepared to study the effects of rolling parameters on the bainitic structure and on the strength-toughness properties. In order to obtain the desired bainitic structure, the reheating temperature should be high enough to allow more NbC to dissolve in austenite. It was also shown that the finish-rolling temperature had a strong effect on the mechanical properties. Both yielding and tensile strengths decreased with decreasing finish-rolling temperature, whereas toughness increased with decreasing finish-rolling temperature. However, the final thickness of the finish-rolling plate was found to have little influence on the strength. It was also found that the steels studied became considerably harder when heated at a temperature of around 600 °C, owing to the precipitation of NbC in a bainitic ferrite matrix.

Continuous heating transformation of bainite to austenite

Abstract The transformation of a bainitic Feue5f8Mnue5f8Siue5f8C alloy into austenite has been studied using dilatometry, transmission electron microscopy and microanalytical techniques. The formation of austenite was investigated using two different starting microstructures, the first consisting of a mixture of bainitic ferrite and residual austenite, and the second of a mixture of tempered bainitic ferrite and carbides. Results from isothermal austenitization experiments confirm earlier work on a different alloy, that because of the incomplete reaction phenomenon associated with bainite growth, there is a large temperature hysteresis before the reverse transformation to austenite becomes possible. Continuous heating experiments revealed an identical austenization behaviour for both initial microstructures when the heating rate utilized was small. This is because any residual austenite then tends to transform into pearlite or to decompose into ferrite and discrete particles of carbides before the sample reaches a temperature where austenite growth becomes thermodynamically feasible. Consequently, the two initial microstructures become identical by the time T γ is reached. At faster heating rates the residual austenite remains stable during heating and then commences to grow as the appropriate elevated temperature is reached. A slightly higher degree of superheating is found to be necessary in the absence of residual austenite in the starting microstructure, since austenite nucleation is then necessary prior to growth. Since the excess superheating is rather small, the results indicate that nucleation does not appear to be a major hurdle to the formation of austenite in the alloy studied.

Cyclic deformation and phase transformation of 6Mo superaustenitic stainless steel

A fatigue behavior analysis was performed on superaustenitic stainless steel UNS S31254 (Avesta Sheffield 254 SMO), which contains about 6wt.% molybdenum, to examine the cyclic hardening/softening trend, hysteresis loops, the degree of hardening, and fatigue life during cyclic straining in the total strain amplitude range from 0.2 to 1.5%. Independent of strain rate, hardening occurs first, followed by softening. The degree of hardening is dependent on the magnitude of strain amplitude. The cyclic stress-strain curve shows material softening. The lower slope of the degree of hardening versus the strain amplitude curve at a high strain rate is attributed to the fast development of dislocation structures and quick saturation. The ε martensite formation, either in band or sheath form, depending on the strain rate, leads to secondary hardening at the high strain amplitude of 1.5%.

Dual ferrite-martensite treatments of a high-strength low-alloy ASTM A588 steel

Dual ferrite-martensite (DFM) treatments of an ASTM A588 steel have been investigated. The treatments consisted of initial austenitization and quenching to form 100% martensite, followed by annealing in the (α + γ) region and subsequent quenching. It was found that DFM microstructures of the steel contained continuous globular martensite along the prior austenite grain boundaries and acicular martensite within the prior austenite grains. The DFM of the steel exhibited the superb combinations of strength and elongation over a range of elongation 15% to 20% in a 3 cm gauge length. For a sample with 18% elongation, the yield strength was 49 kg mm−2 (70 × 103 p.s.i.) and the ultimate tensile strength 68 kg mm−2 (97 × 103 p.s.i.). The type of initial microstructure prior to the dual phase treatment is of prime importance in the determination of the morphology of DFM structure. The results show that the initial martensite structure is more advantageous than the initial pearlite plus proeutectoid ferrite and austenite structures.

Acicular ferrite transformation in deformed austenite of an alloy-steel weld metal

The effect of different amounts (5, 10 and 15%) of compressive deformation of austenite on the isothermal transformation of acicular ferrite in an alloy-steel weld metal has been investigated. It was found that prior deformation of austenite significantly enhanced acicular ferrite transformation. At the same isothermal transformation temperature, as a higher amount of prior deformation was applied, a greater quantity of acicular ferrite could be obtained and the size of acicular ferrite plates became much finer. These results implied that the effective nucleation sites of acicular ferrite increased with increasing amount of prior deformation. The other results also emphasized that the accumulated strain (due to prior deformation of austenite) could trigger acicular ferrite to nucleate on inclusions at high temperatures, where undeformed austenite remained stable. The acicular ferrite start temperature was found to be raised continuously by increasing the amount of prior deformation of austenite. Further evidence suggests that the application of deformation can boost the driving force for acicular ferrite formation. This phenomenon is similar to the case in which martensite forms under the influence of deformation.

The coexistence of acicular ferrite and bainite in an alloy-steel weld metal

For a multirun weld metal the fusion zone can be classified into two main components: the primary zone obtained during cooling of the weld from the liquidus and the secondary microstructure (commonly called the reheated zone) obtained during the deposition of further weld metal. The primary zone usually consists of large columnar austenite grains at high temperatures. On the other hand, the reheated zone of a multirun weld is nominally divided into austenite coarse-grained and fine-grained regions, depending on the peak temperature to which they are exposed. The effect of the grain size of prior austenite on the acicular ferrite transformation is significant. It has been shown [1] that if the primary microstructure of the weld deposit consists essentially of acicular ferrite, then the acicular ferrite can again be obtained in the reheated homogenized weld metal when the austenite grain size is relatively large. However , classical sheaf-like bainitic ferrite can form as the austenite grain size is small. Although Babu and Bhadeshia [2] reported that the acicular ferrite and bainite can coexist (Fig. 1), there