Low-temperature Martensitic Transformation Research Articles

Transient Nucleate Boiling Process Used for Obtaining Super Strong Carbon Steels and Irons

Based on self–regulated thermal process, in the paper four types of thermomechanical treatments are considered. The first is a high temperature thermomechanical treatment (HTTMT) followed by complete martensitic transformation. The second is a low temperature thermomechanical treatment (LTTMT) plus martensitic transformation. The third is the high and low temperature thermomechanical treatment (HTTMT and LTTMT) plus martensitic transformation. And the last includes HTTMT and LTTMT plus bainitic transformation to obtain super strong and ductile materials. It is shown in the paper that listed technologies are enough intensive to obtain very strong and ductile materials using plain high carbon steels. A detailed consideration of all processes in the paper will motivate engineers to perform mentioned technologies in forging shops to receive super strong and ductile materials without costly alloying that saves energy and alloying elements. The paper discusses the opportunity of preventing martensite transformation to receive fine and nano–bainitic microstructure during intensive quenching. A hypothesis is forwarded that explains possible technology used in 8th and 9th centuries in the Middle East to manufacture Damascus steel. The secret of Damascus steel could be the duration of transient nucleate boiling process needed for preventing martensite transformation during forging of steel.

Modelling and design of new stainless-steel welding alloys suitable for low-deformation repairs and restoration processes

The plasticity associated with low-temperature martensitic transformation can be exploited to reduce the stresses developed due to thermal contraction of the weldments. The key feature of stress-mitigating welding alloys is their transformation from austenite to martensite at low temperatures e.g. ideally close to ambient temperature. Thermodynamics databases (MTDATA and SGTE) were used to model and design new welding alloys with low martensitic transformation temperatures (Ms) around 200 °C. The modelling, conducted in this work, was based on this assumption that martensitic transformation starts at a certain temperature when the free-energy change for austenite to transform to ferrite reaches a critical value. Since martensitic transformation is a diffusion-less process, the change in free-energy vs. temperature was calculated for the austenite and ferrite phases with the same composition.Three prototype welding alloys, CamAlloys 4 & 5 and OpenAlloy 1, were successfully designed and made in the University of Cambridge (UK) and the Open University (UK). The design of these alloys was purely based on thermodynamics equations. Comprehensive characterisation, examinations and mechanical tests showed this family of alloys could substantially reduce contraction-induced deformations in stainless steel weldments. One of the applications of these alloys is in the repair and restoration of damaged stainless-steel components.

The Welding of Dissimilar Joints with Low Temperature Martensitic Transformation Filler Material

Abstract The welding of dissimilar joints is very common in systems used in oil exploration and production in deep waters. This commonly involves welding low carbon steel pipes with valves, pumps or other pipes, all of them made of alloy steel. Inconel has been used in such welding as filler material; however, this welding is preceded by a process of buttering in the low alloy steel side and a stress relief heat treatment. An alternative technique presented in this paper is the use of a welding filler metal that, during the cooling process, undergoes a martensitic transformation at a relatively low temperature, so that the resulting deformation processing offsets the contraction that occurs as a result of the cooling process. This study evaluates the application of Maraging 250 weld in a dissimilar joint of the oil industry. Tests were performed on joints of ASTM steels A516 grade 70 and AISI 8630M welded with Maraging 250. The residual stress profiles, hardness, microstructure, tensile strength and impact energy absorbed were evaluated. The welded joint with Maraging 250 showed a high intensity of compressive residual stress. The microstructure of different welding regions was analysed and compared.

Metallurgical principles of microstructure formation in sub-zero treated cold-work tool steels – a review

The beneficial influence of a sub-zero treatment on wear resistant tools and components has been known for over 100 years. On the other hand, the basic metallurgical principles being responsible for enhanced hardness and wear performance, changes in the tempering response and toughness and improved dimensional stability have become known only over the past decade. The sub-zero treatment has, thus, been changed from an art to accepted science. The topic of the current conference paper is the latest theory explaining the metallurgical background for this kind of treatment. This theory states that it is the low-temperature isothermal martensitic transformation that induces secondary microstructural effects, such as an accelerated precipitation rate for transient carbides, formation of very small globular carbides and overall refinement of the microstructure. Consequently, secondary microstructural effects have a clear impact on the most relevant properties. The extent of the improvement or deterioration of these properties may be a result of competitive microstructural effects.

The Properties and Residual Stress of Argon arc Cladding Metal by Low Temperature Martensitic Transformation Powder

Martensitic Fe-based metal was achieved by argon arc surface cladding using self-developed low-temperature phase transformation (LTT) powders. Residual stress (RS), retained austenite (RA) content, hardness and wear resistance of the cladding layer under different powder compositions were tested. Meanwhile, the phase composition, microstructure and wear mechanism of the cladding metal were also analyzed. The results show that under suitable process parameters, the cladding layer and the substrate are metallurgical bonding without crack and porosity. The microstructure of the cladding layer is mainly lath martensite with less RA distributed on. Due to the volume expansion during phase transformation from austenite to martensite at about 200°C degree, large compressive RS is produced within the cladding metals, which has effectively compensated the tensile RS result from thermal shrinkage. The compressive RS reaches maximum of -361 MPa, and the average RA in the cladding layer is 10.85% for powder with composition of 10% Cr and 8% Ni. The hardness of the cladding layer is 2.6 times higher than that of the matrix material, which could reach up to 557.2 HV. The wear resistance of the surface cladding layer is nearly 57 times higher than that of the base material, which has the most excellent comprehensive performance.

Phase transformations in PuGa 1 at.% alloy: New valuable insight into the isothermal martensitic δ→α′ transformation

This paper deals with the results of an investigation of low-temperature martensitic transformation in a metastable PuGa 1 at.% alloy. The kinetics of this process were studied experimentally via in situ XRD characterizations performed during isothermal holds. This revealed a double-C Time–Temperature–Transformation diagram similar to that reported in the literature for enriched alloys. The originality of our results lies in the demonstration of the existence of different martensite morphologies corresponding to the upper and lower parts of the TTT diagram. These microstructural variations seem to be the result of different accommodation mechanisms.The experimental nucleation rate study has suggested an autocatalytic character of the isothermal martensitic nucleation process as well as a large sensitivity to elastic and plastic strain. Indeed, the incubation period and subsequent increase in nucleation rate could be related to the appearance of new embryos rather than the growth of existing plates, while the partial character of the transformation may be attributed to interactions between plates that are being formed in the two-phase δ + α′ alloy. These hypotheses have been confirmed by the classical heterogeneous nucleation approach by Pati and Cohen, which takes into account the autocatalytic character of the transformation. This approach has also indirectly revealed the existence of restraining forces that reflect a high sensitivity of the energy barrier for nucleation to the α′ phase amount.

The effect of thermal cycling on the martensitic transformation in equiatomic CuZr shape memory alloy

This work highlights the lack of stability of the martensitic transformation (MT) upon thermal cycling of equiatomic CuZr shape memory alloy (SMA). The initial MT taking place at high temperature is inhibited within 10 thermal cycles, whilst a low temperature transformation (at −20 °C) is established. The MT evolution is observed through DSC and XRD measurements and SEM observations. Its reversible character and the limited thermal hysteresis suggest a new low-temperature MT in CuZr intermetallic SMA compound.

Characterization of microstructure and residual stress in a 3D H13 tool steel component produced by additive manufacturing

Abstract

Low-temperature martensitic transformation in tool steels in relation to their deep cryogenic treatment

The low-temperature martensitic transformation in steel X153CrMoV12 containing (mass%) 1.55C, 11.90Cr, 0.70V, 0.86Mo is studied using dilatometry, Mössbauer spectroscopy, X-ray diffraction, mechanical spectroscopy and transmission electron microscopy. Some additional measurements were carried out on steel X220CrMoV13-4. It is shown that, in contrast to the widely known absence of martensitic transformation during deep cryogenic treatment, this transformation occurs with isothermal kinetics within the temperature range of −100 down to −170°C with its largest intensity near −150°C. No transformation is observed at −196°C. The remarkable features of the isothermal martensitic transformation are: (i) the plastic deformation, which is explained by the absence of ageing of martensite at low temperatures; and (ii) the abnormally low tetragonality of martensite. In contrast to existing interpretations, the abnormally low c/a ratio is interpreted in terms of the capture of immobile carbon atoms by gliding dislocations during plastic deformation at low temperatures. A recommendation is proposed for optimizing the deep cryogenic treatment of tool steels.

Low-temperature martensitic transformation and deep cryogenic treatment of a tool steel

The tool steel X220CrVMo 13-4 (DIN 1.2380) containing (mass%) 2.2C, 13Cr, 4V, 1Mo and the binary alloy Fe–2.03 mass% C were studied using transmission electron microscopy, Mössbauer spectroscopy, X-ray diffraction and internal friction with the aim of shedding light on processes occurring during deep cryogenic treatment. It is shown that the carbon atoms are essentially immobile at temperatures below −50 °C, whereas carbon clustering in the virgin martensite occurs during heating above this temperature. An increase in the density of dislocations, the capture of immobile carbon atoms by moving dislocations, the strain-induced partial dissolution of the carbide phase, and the abnormally low tetragonality of the virgin martensite are found and interpreted in terms of plastic deformation that occurs during martensitic transformation at low temperatures where the virgin martensite is sufficiently ductile.

Thermophysical properties of coexistent phases of plutonium

Plutonium is the element with the greatest number of allotropic phases. Thermally induced transformations between these phases are typically characterized by thermal hysteresis and incomplete phase reversion. With Ga substitutional in the lattice, low symmetry phases are replaced by a higher symmetry phase. However, the low temperature martensitic phase transformation (δ→α') in Ga stabilized δ-phase Pu is characterized by a region of thermal hysteresis which can reach 200°C in extent. These regions of thermal hysteresis offer a unique opportunity to study thermodynamics in inhomogeneous systems of coexistent phases. The results of thermophysical properties measured for samples of inhomogeneous unalloyed and Ga alloyed Pu will be discussed and compared with similar measurements of their single phase constituents.

A Comparison of Residual Stress in Hammer-Peened, Multi-Pass Steel Welds–A514 (S690Q) and S41500

As part of a program to assess the in-situ weldability and mechanical performance of the candidate high strength low alloy A514 (S690Q) steel as an alternative to the S41500 martensitic stainless steel for hydro-turbines, three aspects of the welds were studied: residual stress, Charpy toughness and cavitation erosion resistance. The experimental set-up involved robotized gas metal arc welding (GMAW), performed on U-groove and double-V weld preparation cut into 50 and 75 mm-thick steel plates. Half of the welds were robotically hammer-peened after each weld layer, except for the root pass. Strain gauges measured longitudinal and transverse strains during welding and hammer-peening. Once the weld cooled down to room temperature, the strain gauges provided the surface residual stress level at their location. Two-dimensional, sub-surface, longitudinal, residual stress distributions were measured on cut sections with the contour method, using an optical profilometer. The results showed that hammer-peening completely eliminates the near-surface tensile welding residual stress on the A514 steel, whereas on the S41500 steel, the process is less useful due to the already beneficial effect of the low temperature martensitic transformation during weld cooling. Furthermore, hammer-peening the last weld layer confines tensile residual stress inside the weld, while inducing compressive stress at the weld surface. Charpy test results showed that the A514 weld presented better toughness than the S41500 weld and comparable cavitation erosion resistance. Finally, hammer-peening showed a beneficial effect on cavitation resistance of the weld surface.

Mechanical response of thin SMAW arc welded structures: Experimental and numerical investigation

The mechanical response of thin welded plates made of Ck45 steel during shielded metal arc welding (SMAW) is investigated by employing data from numerical results using the finite element method and laboratory experimental measurements. The aim of this paper is to investigate the effect of the thermal uncertainty encountered during the thermal numerical analysis on the out-of-plane angular distortion of the welded panels. For this purpose a comparison between the numerical mechanical response (using the calibrated and as-predicted thermal history) and the experimental results is carried out. The effect of the microstructural transformations on the angular out-of-plane distortion is also investigated by introducing the material transformation within the numerical analysis by the simplest means in order to clearly demonstrate its influence. The same strategy is followed in order to investigate the effect of low-temperature martensitic transformation on the longitudinal residual stress field by properly combining experimental and numerical results and analyzing an idealized welding fabrication. Discussion is, finally, carried out regarding the design process of welded marine structures.

Ambient-temperature Conditioning as a Probe of Double-C Transformation Mechanisms in Pu-2.0 at. % Ga

AbstractThe gallium-stabilized Pu-2.0 at. % Ga alloy undergoes a partial or incomplete low-temperature martensitic transformation from the metastable δ phase to the gallium-containing, monoclinic α′ phase near -100 °C. This transformation has been shown to occur isothermally and it displays anomalous double-C kinetics in a time-temperature-transformation (TTT) diagram, where two nose temperatures anchoring an upper- and lower-C describe minima in the time for the initiation of transformation. The underlying mechanisms responsible for the double-C behavior are currently unresolved, although recent experiments suggest that a conditioning treatment—wherein, following an anneal at 375 °C, the sample is held at a sub-anneal temperature for a period of time—significantly influences the upper-C of the TTT diagram. As such, elucidating the effects of the conditioning treatment upon the δ⟶α′ transformation can provide valuable insights into the fundamental mechanisms governing the double-C kinetics of the transition. Following a high-temperature anneal, a differential scanning calorimeter (DSC) was used to establish an optimal conditioning curve that depicts the amount of α′ formed during the transformation as a function of conditioning temperature for a specified time. With the optimal conditioning curve as a baseline, the DSC was used to explore the circumstances under which the effects of the conditioning treatment were destroyed, resulting in little or no transformation.

Study of the stability and decomposition process of the β phase in Cu–Al–Ni shape memory alloys

The influence of precipitation processes on the characteristics of the martensitic transformation in two Cu–Al–Ni shape memory alloys processed by powder metallurgy has been analyzed. The Cu–14.37Al–4.2Ni (wt.%) alloy with a hypereutectoid composition undergoes a low-temperature martensitic transformation, M s < 100 °C. On the other hand, the Cu–14.15Al–3.25Ni (wt.%) near eutectoid composition undergoes a high-temperature martensitic transformation, M s > 110 °C. The evolution of the kind of induced martensite, hysteresis and transformation temperatures as a function of thermal history has been studied by differential scanning calorimetry. Also, resonant ultrasound spectroscopy allows changes to be monitored of elastic constants during the evolution of the microstructure. Finally, a comparative analysis of the behavior of both alloys has been made.

Vibrational entropy of the γ−α martensitic transformation in Fe71Ni29

The low-temperature martensitic phase transformation in Fe71Ni29 was studied by X-ray diffractometry, differential scanning calorimetry (DSC) and inelastic neutron scattering. From inelastic neutron scattering spectra, the phonon density of states of the high-temperature austenite and low-temperature martensite were obtained, showing a substantial stiffening in the martensite phase. The change in vibrational entropy associated with this stiffening was found to be . Using published results for the Fe partial density of states in disordered BCC Fe–Ni of similar composition, a Ni partial density of states was obtained, which shows a significant number of modes at much lower energies than its Fe counterpart. The martensitic and reverse-martensitic transformations occur at different temperatures and the total entropies of transformation were measured by DSC for both. From these measurements and published magnetisation results for this alloy, it is found that the change in magnetic entropy is twice as large as the phonon contribution in the reverse martensitic transformation, accounting for the inequality of the total entropies of the forward and reverse transformations.

Bainite and stress-induced martensite in an AISI type 300 steel: an X-ray diffraction study of the microstructure by the Rietveld method

The present study concerns the X-ray characterization of the microstructure of an AISI type 300 stainless steel with composition Fe–17Cr–8Ni–1Mn–0.1C (wt%), undergoing stress-induced martensite and bainite transformations following γ(f.c.c.) → α(b.c.c.) transformation reactions. The methodology applied is a Rietveld whole X-ray profile fitting technique, adopting the recently developed softwareMAUD(Materials Analysis Using Diffraction) which incorporates the Popa model for crystallite size and microstrain [root mean square (r.m.s.) strain] and the preferred orientation of the crystallites. The analysis also considers the lattice-defect-related features of the evolved microstructures, namely stacking-fault probabilities and dislocation-density values. The study revealed a high degree of stress-induced martensitic transformation (∼94%) in the cold-worked (hand-filed powder) state of the material. However, no low-temperature martensitic transformation could be observed in this class of materials at temperatures below the martensite start temperature (Ms). On isothermal aging at 673 K for different periods of time (1, 3 and 6 h), the austenite transforms partially into bainite with continuously varying volume fractions (17, 42 and 44%) and the corresponding microhardness values are 218, 230 and 233 kg mm−2. The size–strain–shape analysis adopting the Popa model reveals anisotropic and almost isotropic values of the crystallite sizes and r.m.s. strains in the austenite and bainite microstructures, respectively. High values of the dislocation density (∼1016 m−2) were observed in the bainite due to the plastic deformation of the austenite induced by the shape change of the growing bainite. The values of all the above defect parameters have been evaluated and compared in order to elucidate the structure–property relationships.

Low temperature martensitic transformation of zirconia with low contents of yttria

Small spherical specimens of the tetragonal phase were obtained for ZrO 2 -1.40∼1.60mol% Y 2 O 3 . These specimens transformed into the stable monoclinic phase, either isothermally or athermally depending on being heated or subzero cooled, respectively. In this paper, the characteristic of the latter transformation was studied. The results include the dependence of M b temperature on the composition, grain size, and specimen size. Other characteristics of the martensitic transformation were also presented.

The martensite phase of high-purity lithium

The low-temperature martensitic transformation of pure lithium is characterized by a complex phase diagram exhibiting the coexistence of several close-packed phases with the bcc matrix. On cooling, below the martensitic start temperature a long-range ordered 9R structure and a one-dimensionally disordered polytype phase are observed. On heating, an intermediate fcc phase appears in the hysteresis regime. Microstructure investigations, application of the crystallographic theory of martensite and thermal cycling during neutron scattering experiments together strongly indicate that fcc is the true thermal equilibrium phase. The 9R structure is a metastable phase which minimizes the lattice mismatch with respect to the bcc matrix and therefore is formed for geometrical reasons.

Effects of thermocapillary forces during welding of 316L-type wrought, cast and powder metallurgy austenitic stainless steels

The Large Hadron Collider (LHC) is now under construction at the European Organisation for Nuclear Research (CERN). This 27 km long accelerator requires 1248 superconducting dipole magnets operating at 1.9 K. The cold mass of the dipole magnets is closed by a shrinking cylinder and two end covers at both extremities of the cylinder. The end covers are welded, cast or powder metallurgy (PM) dished-heads equipped with a number of protruding nozzles for the passage of the different cryogenic lines. Structural materials and welds must retain high strength and toughness at cryogenic temperature. AISI 316L-type austenitic stainless steel grades have been selected because of their mechanical properties, ductility, weldability and stability of the austenitic phase against low-temperature spontaneous martensitic transformation. 316LN is chosen for the fabrication of the end covers, while the interconnection components to be welded on the protruding nozzles will be fabricated from forged 316L or 316LN, and welded 316L tubes. Autogenous welds between the nozzles and the interconnection components will be performed by the automatic orbital TIG technique. Several thousands of welds are foreseen. When welding together grades of slightly different composition, or grades issued from different fabrication methods (cast, PM, cold or hot rolled, forged, etc.), phenomena such as variable weld penetration, “off-centre welding” and “arc wander” may possibly appear, resulting in uncontrolled formation of non-axisymmetric welds. Such deflections of the weld pool are difficult to correct for an automatic process and may affect the soundness of the weld. A large and systematic campaign of welding tests associated with video recording of the melt pool has been carried out. Hot metal deflections have been precisely quantified. The results are interpreted in terms of the different content of soluble surface-active elements of the various steel batches and the directions of the thermocapillary flow arising from these different contents. This interpretation gives a quantitative prevision of the hot metal deflections. Possible corrections applicable to automatic welding processes are discussed.