Heat Treatment Response Research Articles

Tuning austenite stability through prior microstructure control in a low-alloy Q&P steel

Quenching and partitioning (Q&P) processing is a widely accepted heat treatment methodology for creating high strength steels consisting of ferrite, martensite, and austenite, while maintaining relatively low manufacturing costs. Though the research on effects of prior microstructure is limited, an understanding of the heat treatment response of different starting microstructures is critical to processing and creating steels with complex microstructures that contain retained austenite and may afford opportunities to further optimize properties. This study investigates the influence of starting microstructure (ferrite/pearlite versus martensite) and prior levels of cold work (38 verses 58 %) on the microstructural development and mechanical properties of a 0.2 C-2.0 Mn-1.5 Si (wt.%) steel exposed to Q&P processing. Samples with a starting martensitic microstructure resulted in higher retained austenite fractions and a more homogeneous microstructure after Q&P processing compared to a starting microstructure of ferrite-pearlite. Starting martensitic microstructures also displayed higher work hardening rates and higher uniform elongations. Larger cold reductions saw accelerated dissolution kinetics and austenite formation during intercritical annealing, resulting in more similar final microstructures from the ferrite-pearlite and martensitic starting microstructures. The results presented here indicate that varying prior processing can be a route to manipulate and control austenite stability in a Q&P processed steel.

Laser powder bed fusion of a β titanium alloy: Microstructural development, post-processing, and mechanical behaviour

β-Titanium alloys demonstrate unique properties, especially their low elastic modulus, good balance of strength and ductility, and strong heat treatment response. In this study, pre-alloyed powder of a β-Ti-alloy Ti–34Nb–13Ta–5Zr-0.3O (TNTZO) was processed using Laser Powder Bed Fusion (LPBF). Dense builds that demonstrate a full β microstructure with limited texture were achieved following process optimisation, with an elastic modulus of 56.5 GPa, tensile strength of 756 MPa, elongation-to-failure of 20 %, and recoverable strain of 1.3 %. Due to the high oxygen content, the β→α′′ deformation induced transformation was supressed, which resulted in a typical elastic-plastic stress-strain behaviour unlike the double-yielding behaviour typically experienced in superelastic Ti-alloys. The alloy was particularly sensitive to ageing heat treatments that resulted in needle α-precipitation, with tensile strengths exceeding 950 MPa, elastic modulus of ∼80 GPa, and recoverable strain of 1.5 %, yet at the expense of a reduced elongation-to-failure of 9 %. Transmission electron microscopy and electron backscattered diffraction investigations revealed the deformation mechanism was slip-dominated with no trace of twinning or stress induced phase transformation.

Processing, Microstructure Evolution, and Heat Treatment Response of AA2024 and Its Metal Matrix Composites of In Situ TiB2 Dispersoids

Processing, Microstructure Evolution, and Heat Treatment Response of AA2024 and Its Metal Matrix Composites of In Situ TiB2 Dispersoids

The clinical value of heat shock protein 90α in predicting the prognosis of interventional therapy for hepatocellular carcinoma

Objective: To evaluate the relationship between plasma heat shock protein 90α (HSP90α) levels and treatment response after four weeks and long-term prognosis after transarterial chemoembolization (TACE) of hepatocellular carcinoma (HCC). Methods: The clinical data of HCC patients who underwent TACE in the Department of Interventional Radiology, Cancer Hospital of Chinese Academy of Medical Sciences from August 2017 to December 2018 were retrospectively collected. Chi-square tests were used to analyze the relationship between plasma HSP90α level and clinicopathological features before TACE treatment. Univariate and multivariate logistic regression analysis was used to analyze the influencing factors of TACE treatment response. Univariate and multivariate Cox regression analysis was used to analyze the influencing factors of progression-free survival (PFS) after TACE treatment. Results: The expression level of plasma HSP90α in 96 patients before TACE treatment was (99.70 ± 66.61) ng/ml. Compared with the low HSP90α group (n=66), the high HSP90α group (n=30) had larger tumors, higher alpha-fetoprotein enrichment, more positive vascular invasions, and more advanced Barcelona Clinic Liver Cancer (BCLC) stages (all P<0.05). After four weeks of TACE treatment, 41 patients in the response group and 55 patients in the non-response group were evaluated. The difference of HSP90α expression levels between the response group and the non-response group before and after TACE treatment was (-32.20±22.79) ng/ml and (7.20±51.94) ng/ml, respectively, and the difference was statistically significant (P<0.001). Multivariate logistic regression analysis showed that Child-Pugh classification (OR=0.186, P=0.046), vascular invasion (OR=0.132, P=0.025), and the percentage reduction of plasma HSP90α after TACE treatment (percentage reduction 25%-50%: OR=5.061, P=0.013; percentage reduction >50%: OR= 86.831, P<0.001) were independent influencing factors for the response to TACE treatment in HCC. The median PFS of the 96 patients was 8.7 months. Multivariate Cox regression analysis showed that BCLC stage (stage B: HR=2.804, P=0.008; stage C: HR=4.628, P<0.001) and the percentage reduction of plasma HSP90α after TACE treatment (percentage reduction 25%-50%: HR=0.569, P=0.051; percentage reduction >50%: HR=0.198, P<0.001) were independent influence factors for the PFS in these HCC patients after TACE treatment. Conclusion: Plasma HSP90α may represent a novel biomarker for predicting efficacy of TACE and PFS of patients with HCC.

Effect of compositional variations on the heat treatment response in 17-4 PH stainless steel fabricated by laser powder bed fusion

17–4 precipitate hardening (PH) stainless steel is used in various applications including in the aerospace, marine, and chemical industries, largely due to its unique combination of corrosion resistance and high strength, which is achieved by the formation of nanoscale Cu-rich precipitates during aging. 17–4 PH has been widely researched for its applicability for laser powder bed fusion (LPBF). However, there are discrepancies in the literature on its heat treatment response, which seem to be linked to compositional variations. Systematic studies of the interplay between these variations and nanoscale precipitation are currently missing. Using atom probe tomography, we present a systematic study of the heat treatment responses of two variants of LPBF 17–4 PH builds fabricated from different powder feedstocks, with significant differences in N contents (High vs Low N 17–4). Both variants formed predominantly δ-ferritic as-built microstructures. The as-built High N 17–4 variant showed a higher volume fraction of austenite which further increased upon solution annealing and quenching. The consequence was no appreciable hardening effect due to the absence of Cu precipitation in either austenite or martensite after aging, degrading the alloy's desirable property profile. Conversely, Low N 17–4 showed no austenite in the as-built condition and a fully martensitic matrix after solution annealing. This variant had the desired aging response; a ∼ 140 HV 5 increase in hardness due to nanoscale Cu precipitation. Our findings describe the deleterious effects of compositional variations incurred during the LPBF process flow and how they can be overcome in 17–4 PH and similar steels.

Heat treatment response and mechanical properties of a Zr-modified AA2618 aluminum alloy fabricated by laser powder bed fusion

Additive manufacturing of commercial materials designed for casting is challenging, particularly for hot-crack-susceptible alloys. Adding Zr to Al alloys has a positive effect on part consolidation by suppressing hot cracks and enabling nanoprecipitation hardening. Despite the special solidification conditions during laser powder bed fusion and the use of modified alloy chemistries, the heat treatments that were optimized decades ago for cast or wrought alloys are still applied. These treatments thus do not optimally exploit the full potential of novel alloys tailored for additive manufacturing. This work investigates the microstructure, precipitate formation, and mechanical properties of a Zr-modified 2618 Al alloy via laser powder bed fusion. A three-step heat treatment yields several nanometric phases, including L12-Al3Zr, S-Al2CuMg, and Mg2Si, which strengthen the alloy. Tensile tests reveal ultimate and yield strengths of 478 ± 7 MPa and 401 ± 3 MPa, respectively, and an elongation to fracture of 9.2 ± 1.1%. These values match conventionally manufactured and heat-treated 2618 standards and exceed T6 properties. The results emphasize the need to adapt heat treatments for additive manufacturing, not just materials.

Microstructure Evolution and Mechanical Properties of Al-Cu-Mg Alloys with Si Addition.

The aim of this study was to investigate the impact of the addition of a minor quantity of Si on the microstructure evolution, heat treatment response, and mechanical properties of the Al-4.5Cu-0.15Ti-3.0Mg alloy. The microstructure analysis of the base alloy revealed the presence of α-Al grains, eutectic α-Al-Al2CuMg (S) phases, and Mg32(Al, Cu)49 (T) phases within the Al grains. In contrast, the Si-added alloy featured the eutectic α-Al-Mg2Si phases, eutectic α-Al-S-Mg2Si, and Ti-Si-based intermetallic compounds in addition to the aforementioned phases. The study found that the Si-added alloy had a greater quantity of T phase in comparison to the base alloy, which was attributed to the promotion of T phase precipitation facilitated by the inclusion of Si. Additionally, Si facilitated the formation of S phase during aging treatment, thereby accelerating the precipitation-hardening response of the Si-added alloy. The as-cast temper of the base alloy displayed a yield strength of roughly 153 MPa, which increased to 170 MPa in the Si-added alloy. As a result of the aging treatment, both alloys exhibited a notable increase in tensile strength, which was ascribed to the precipitation of S phases. In the T6 temper, the base alloy exhibited a yield strength of 270 MPa, while the Si-added alloy exhibited a significantly higher yield strength of 324 MPa. This novel Si-added alloy demonstrated superior tensile properties compared to many commercially available high-Mg-added Al-Cu-Mg alloys, making it a potential replacement for such alloys in various applications within the aerospace and automotive industries.

Heat treatment response of additively manufactured 17-4PH stainless steel

Heat treatment response of additively manufactured 17-4PH stainless steel

Multi-length-scale study on the heat treatment response to supersaturated nickel-based superalloys: Precipitation reactions and incipient recrystallisation

A supersaturated γ phase microstructure is produced in Ni-based superalloys using laser powder bed fusion (L-PBF) – the cooling rate arising from the process is shown to suppress the solid-state precipitation of the γ′ phase. The response of the material to a heat treatment therefore requires new understanding at the fundamental level, since the first population of γ′ precipitate forms upon heating, in contrast to cooling from homogenisation above the γ′ solvus. Here, we have interrogated two new nickel-based superalloys designed for the L-PBF technology, both in situ and ex situ, at multiple length scales using advanced characterisation methods. First, we conducted in situ synchrotron X-ray diffraction during various heat treatments to trace the evolution of the γ′ volume fraction with temperature. The first structural changes were detected at an unexpectedly low temperature of ∼445 °C. Second, the temperature for γ′ nucleation and its sensitivity to heating rate was studied using an electrical resistivity method. Then, the γ′ composition upon heating, isothermal holding and cooling is analysed using atom probe tomography (APT), the result is rationalised by further scanning-transmission electron microscopy and nanoscale secondary ion mass spectroscopy. Finally, static recrystallisation during isothermal exposure was investigated, which occurs within minutes. This work sheds light on a new strategy of tailoring microstructure for additively manufactured superalloys by manipulation of the γ′ precipitate distribution upon heating.

Developing a neural network to model the mechanical properties of 13-8 PH stainless steel alloy

We systematically evaluated the effects of raw material composition, heat treatment, and mechanical properties on 13-8PH stainless steel alloy. 13-8PH is widely used in the aerospace industry for various structural components. Little is known about the predictive modeling of mechanical properties for 13-8 PH per heat treatment. To achieve the desired properties, multiple back-and-forth heat treatments and mechanical tests are often required which utilize significant amounts of resources. Thus, we hypothesized that no specific heat treatment temperature can be used to achieve the desired property ranges across all 13-8PH compositions. We heat treated 13-8PH alloy through solution treatment, cryogenic treatment, and final elevated temperature aging. We conducted double shear testing to measure this mechanical property. Consistent with expectations, experimental data confirmed that no single specific aging heat treatment temperature could be used to achieve desired property range for all compositions. A unique aging heat treatment temperature is required and there is a need to develop a model to predict heat treatment response for each composition. The results of the neural network models were in agreement with our experimental results and aided in the evaluation of the effects of aging temperature on double shear strength. The data suggests that this model can be used to determine the appropriate 13-8PH alloy aging temperature needed to achieve the desired mechanical properties, eliminating the need for many costly trials and errors through re-heat treatments.

Influence of cooling condition after solidification on T5 heat treatment response of hypoeutectic Al-7Si-0.4Mg casting alloy

The effect of the cooling conditions after solidification of a hypoeutectic Al-7Si-0.4Mg alloy on the precipitation behavior during T5 heat treatment (artificial aging immediately after casting) was investigated. Al-7Si-0.4Mg alloys with two different cooling conditions, mold cooling and water quenching, were respectively fabricated by holding the sample in a permanent mold throughout solidification and by placing the sample in room temperature water just after solidification was completed. TEM analysis of the as-cast alloys confirmed that mold cooling after solidification led to the formation of a small amount of clusters, Si and B′ in the mold which may have pre-precipitated during cooling, increasing the hardness and strength of the as-cast alloy. However, the hardenability of the alloy with mold cooling during T5 heat treatment is less pronounced than the alloy with water quenching. This suggests that the pre-precipitation in the alloy with mold cooling suppresses the aging hardening response, reducing the overall strength of the alloy with T5 heat treatment compared to that of the alloy with no pre-precipitation. Analysis of precipitation kinetics and number densities of precipitates shows that the pre-precipitation can interfere with the precipitation of β", otherwise promoting the precipitation of coarse β', U2 and Si, which reduced overall T5 precipitation hardenability. Therefore, for the T5 heat treatment condition, we suggest that the cooling conditions after solidification are a key factor affecting the best performance of precipitation hardening.

Heat treatment response and characterization of Ti6Al4V + xMo produced by laser metal deposition

During the laser metal deposition additive manufacturing processing of Ti6Al4V ELI alloy, the parts produced were exposed to high levels of thermal gradients, which resulted from rapid heating and cooling rates in the material. This had an adverse effect on the material properties, as tensile residual stresses were created in the parts and increased the strength while significantly reducing ductility. Additionally, the presence of columnar grains compromised the material properties because it resulted in inhomogeneous microstructures that exhibit anisotropy in parts. This study investigated the influence of β annealing temperatures on the microstructure of Ti6Al4V ELI alloy produced during laser metal deposition, and the Ti6Al4V ELI in-situ alloyed with varying molybdenum content. The observations made included a temperature driven phase transformation, which resulted in a change from columnar to equiaxed grains due to heat treatment of the Ti6Al4VELI alloy, while the solidification structure of the alloy changed from planar to cellular due to the addition of Mo. The Ti6Al4V ELI alloy heat treated at 1000 °C reported a hardness profile of 204 ± 5HV0.3, which was comparable to the reported hardness (206 ± 34 HV0.3) of the Ti6Al4V ELI in-situ alloyed with 10 mass percent Mo (10% Mo). This implies that the effects of the in-situ alloying of Ti6Al4VELI with 10% Mo are comparable to the heat treatment of Ti6Al4V ELI alloy at a β annealing temperature of 1000 °C, in terms of stabilization of the β-phase.

Multi-Length-Scale Study on the Heat Treatment Response to Supersaturated Nickel-Based Superalloys: Precipitation Reactions and Incipient Recrystallisation

Multi-Length-Scale Study on the Heat Treatment Response to Supersaturated Nickel-Based Superalloys: Precipitation Reactions and Incipient Recrystallisation

On the Formation Mechanism of Banded Microstructures in Electron Beam Melted Ti–48Al–2Cr–2Nb and the Design of Heat Treatments as Remedial Action

The formation mechanism of banded microstructures of an electron beam melted engineering intermetallic Ti–48Al–2Cr–2Nb alloy, the solidification behavior, and the heat treatment response are investigated via a process parameter study. Scanning electron microscopy, hardness testing, X‐ray diffraction, electron probe microanalysis, thermomechanical analysis, electron backscatter diffraction, heat treatments, as well as thermodynamic equilibrium calculation, and numerical simulation were performed. All specimens show near‐γ microstructures with low amounts of α2 and traces of βo. Fabrication with an increased energy input leads to an increased Al loss due to evaporation, a lower α‐transus temperature, and to a higher hardness. Banded microstructures form due to abnormal grain growth toward the bottom of original melt pools, whereas α2 in Al‐depleted zones enables a Zener pinning of the γ‐grain boundaries, leading to fine‐grained areas. Via numerical simulation, it is shown that increasing the energy input leads to larger maximum temperatures and melt pool sizes, longer times in the liquid state, and more remelting events. Solidification happens via the α‐phase and increasing the energy input leads to an alignment of (111)γ in building direction. Furthermore, banded microstructures respond heterogeneously to heat treatments. Heat treatment is introduced based on homogenization via phase transformation to obtain isotropic microstructures.

Thermal stability and influence of Y2O3 dispersoids on the heat treatment response of an additively manufactured ODS Ni–Cr–Al–Ti γ/γ′ superalloy

The post-processing treatment response of a Y2O3 oxide-dispersion-strengthened (ODS) and γ/γ′-strengthened Ni–8Cr–5.5Al–1Ti (wt.%) model alloy, produced by laser powder bed fusion (L-PBF) is studied. The solutionizing treatment at 1260 °C induces significant recrystallization of the initial elongated grain microstructure, characteristic of L-PBF processing. Grain-boundary pinning by dispersoids affects the recrystallization efficiency, leaving behind pockets of fine 1–10 μm grains in-between 200–400 μm large grains with complex shapes. The Y2O3/Y2O2S dispersoids exhibit excellent coarsening resistance during solutionizing, and are stable during subsequent aging heat treatments at 850 °C. The slow cooling from solutionizing promotes the formation of secondary and tertiary γ′ precipitates, with the ODS particles located within the γ channels. Within the timescale of the isothermal aging, up to ∼1000 h, the coarsening rate of the γ′ precipitates is slightly faster in the ODS material. This allows to independently optimize the γ′ volume fraction and size for balanced properties. Additionally, the capability of hot isostatic pressing (HIP) to close and heal cracks is investigated under various consolidation parameters. This post-process treatment allows to significantly widen the L-PBF processing window of the alloy.

Thermochemical surface hardening of Ti-6Al-4V: On the role of temperature and treatment media

The present work addresses the surface hardening and heat treatment response of Ti-6Al-4V grades G5 and G23 on thermochemical surface treatment in different temperature regimes as well as in different gaseous and plasma-based media. Grades G5 and G23 were subjected to gaseous surface hardening using various gas compositions and temperature regimes. Two different series of gaseous surface treatments were carried out: 1) Carbo-oxidizing of G23 in mill-annealed condition was performed in CO gas at (high) temperatures ranging from 777 to 1027 °C. Treatment at 1027 °C resulted in the deepest case with the formation of a thin surface layer of titanium sub-oxides supported by a thick TiC x O 1-x layer as the dominant phase. Post-nitriding of the carbo-oxidized specimen raised the hardness of the TiC x O 1-x phase by the incorporation of nitrogen, yielding values up to ~2900 HV and multi-layered structures. 2) Carbo-oxidizing of G5 with a bi-modal microstructure in CO/CO 2 gas mixture at (intermediate) temperatures in the range 677–777 °C. Treatment at 777 °C resulted in a surface hardness of ~1900 HV and a ~60 μm-thick diffusion zone. In addition to gaseous surface hardening, also plasma-assisted methods were applied and compared to their gaseous counterparts. Intermediate- and low-temperature plasma treatments were carried out on G23: 3) Plasma (carbo-nitro)oxidizing was performed in a CO 2 and N 2 gas mixture at 750 °C and (carbo-)oxidizing in CO 2 gas at 650 °C. 4) Plasma nitriding was conducted in a N 2 /H 2 gas mixture at 750 °C and 850 °C. The widely different types of treatments applied in this work represent the diversity of the gaseous and plasma-based thermochemical methods that can be applied for surface engineering of titanium alloys . • Surface hardening of titanium is feasible in widely different temperature regimes. • Hard δ-TiC x O 1-x requires a high a C to form at high temperatures. • High p O 2 maintains a high hardness value in the expanded α at lower temperatures. • Plasma nitriding reduces the required temperature for the formation of hard nitrides. • Temperature is the key factor for surface hardening using CO 2 .

Differential Heat Treatment Response of Cast plus Homogenized and Forged Billets of Aluminum Alloy AA7075

Differential Heat Treatment Response of Cast plus Homogenized and Forged Billets of Aluminum Alloy AA7075

Impact of retained austenite on the aging response of additively manufactured 17-4 PH grade stainless steel

Additively manufactured precipitation hardened (PH) martensitic grade stainless steels display wide variations in their heat treatment response and mechanical properties. Much of this variability can be attributed to the selection of the atomizing gas and the resulting variations in powder composition. Differences in nitrogen compositions present in argon and nitrogen atomized powders led to the formation of a wide range of retained austenite levels between 0 and 81 vol% and significant differences in mechanical properties in as-deposited and aged 17-4 PH grade stainless steels. Material fabricated using argon atomized powder contained little to no retained austenite and displayed mechanical properties in the peak-aged condition similar to those in wrought alloys (1359 ± 59 MPa ultimate tensile strength and 10.1 ± 0.8% elongation to failure). On the other hand, materials fabricated with nitrogen atomized powder displayed between 20% and 81% retained austenite, leading to discontinuous yielding driven by the strain-induced transformation of the retained austenite to martensite. Even with the presence of this discontinuous yielding, the materials fabricated using the nitrogen atomized powders consistently displayed strengths 300 MPa higher in the as-deposited, 200 MPa higher in the peak-aged, and 500 MPa higher in the over-aged conditions than materials fabricated using the argon atomized powders. By directly comparing the local strains and microstructures in nitrogen atomized materials, a [011]γ//[001]α relationship in line with the Bain transformation process was present between the retained austenite and transformed martensite across the as-deposited and aged conditions.

Features of A356 alloy solidified under water cooling conditions

This study focused on using a simple quench method to study the effect of ablation water cooling on microstructural features and heat treatment response of A356 alloy. Molten alloy of a given quantity was solidified in water and the resultant microstructure was characterized and compared with that solidified in metal molds and that reported on ablation castings. It is found that the simple method is suitable in simulating certain features of eutectic solidification during ablation casting. The microstructural length scales obtained in this multicomponent alloy are comparable to that predicted by theories developed for binary alloys. The heat treatment response of the water cooled samples are much faster than that of conventional castings which can be translated into reduced energy consumptions and significant cost savings if the heat treating process is optimized for ablation casting.

Influence of Si Content on Tensile Properties and Fractography of Al-Mg-Si Ternary Alloys.

This study investigated the heat treatment response and tensile properties of Al-6 mass%Mg-xSi (x = 1, 3, 5, and 7 mass%) ternary alloys. Further, the fracture behavior of these alloys in response to heat treatment for different temper conditions was also examined. Scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS) analysis of the as-cast alloys revealed, in all of them, the presence of iron-bearing phases (in a size range of 10˜60 μm) that did not dissolve or become refined upon heat treatment. Additionally, eutectic Mg₂Si and Al₃Mg₂ phases were found in Alloy I (Al-6Mg-1Si), while eutectic Mg₂Si and Si phases were found in the rest of the alloys. In the as-cast condition, the tensile properties of the examined alloys decreased in relation to increasing Si content. Nonetheless, after heat treatment, the yield strength of the alloys with high Si content (>3 mass%) increased significantly compared with that in the as-cast condition. A yield strength greater than 300 MPa was achieved in both Alloy III (Al-6Mg-5Si) and Alloy IV (Al-6Mg-7Si), although this was achieved at the expense of ductility. According to the fractography of the tensile-fractured surfaces undertaken using optical and scanning electron microscopy, fractures of the iron-bearing phases were found to be the source of cracking in alloys with high Si content. In the case of those with low Si content (≤3 mass%), cracks were believed to have been caused by the debonding of iron-bearing phases from the aluminum matrix.