Partial Solution Treatment Research Articles

From as-cast to heat treated X-40 superalloy: Effect of cooling rate after partial solution treatment on microstructural evolutions and mechanical properties

Abstract This research study aims at investigating the influence of partial solution treatment and the subsequent cooling strategy on the microstructure and mechanical properties of an as-cast Co–Cr–W superalloy. Three different cooling scenarios were employed to explore the effect of cooling rate on the carbide reprecipitation potential from the \solid solution formed during the heat treatment. The partial dissolution/breakdown of the continuous network of the Cr-rich M7C3 carbides during the partial solution treatment cycle along with their transformation to M23C6 carbides and formation of secondary carbides during the cooling stage were taken into account to discuss the obtained hardness values, and high-temperature creep results. Regardless of the applied cooling rate, the secondary carbides were formed during the cooling stage, and the heat-treated samples showed almost the same microstructure and mechanical properties, which eliminates the need for the aging cycle. This can alleviate the concerns associated with the heat treatment of the large-size X-40 industrial components since the gradient in the cooling rate after the heat treatment cycle would not lead to non-uniform parts. From the hardness and creep viewpoints, all of the samples could meet the minimum requirements of the X-40 superalloy. Mechanical properties suggest that the applied partial solution treatment cycle can be considered as a suitable candidate for high-temperature applications of X-40 superalloy.

Effect of Partial Solution Treatment Temperature on Microstructure and Tensile Properties of 440C Martensitic Stainless Steel

The 440C martensitic stainless steel is considered to be among the hardest steels, owing to its high carbon content. Careful heat treatment of this material introduces multiple carbide particles, which can alter microstructure and mechanical properties. This study focused on the effect of austenitisation temperature on the microstructure and tensile properties of 440C steel. Austenitisation was performed on the austenite + carbide region, because 440C steel lacks a single-phase region. The steel was austenitised at two different temperatures; namely, 1160 °C and 950 °C, and subjected to oil quenching. The as-quenched samples showed a typical lath martensite structure with retained austenite phase. The treatments at 1160 °C and 950 °C promoted the formation of M7C3 and M23C6 carbides, respectively. The austenite grains in the sample treated at 1160 °C showed a higher growth rate than those in the sample treated at 950 °C. The sample treated at 1160 °C showed low-fraction and a large-size carbide phase. The Zener pinning force decreased, thereby increasing the austenite grain growth in the sample treated at 1160 °C. The hardness and 0.2% proof stress of the sample treated at 950 °C were higher than those of the sample treated at 1160 °C, owing to the higher martensite content in the former. The strength–ductility balance of the sample treated at 950 °C was higher than that of the sample treated at 1160 °C. The decreased austenitisation temperature resulted in improved mechanical properties of the steel. Therefore, the austenitisation temperature alters the microstructure and mechanical properties of 440C steel.

Insight into the partial solutionisation of a high pressure die-cast Al-Mg-Zn-Si alloy for mechanical property enhancement

A partial solution treatment was investigated to enhance the mechanical properties for a die-cast Al-Mg-Zn-Si alloy. The strengthening mechanism was identified that the Mg32(Al, Zn)49 intermetallics and equilibrium η-MgZn2 phase formed in the as-cast microstructure were partially dissolved into the α-Al matrix during solutionising and the fine semi-coherent η'-MgZn2 phase was precipitated during subsequent aging. Consequently, the unique microstructure was characterised by the co-existence of equilibrium η-MgZn2 phase and metastable η'-MgZn2 phase in the α-Al matrix, together with the un-changed Mg2Si eutectic and remnant Mg32(Al, Zn)49 intermetallics in the Al-Mg-Zn-Si alloy.

The effects of solution treatment on the microstructure of the cast Ni-based IN100 superalloy

In this research, the effects of the partial, full and partial + full solution heat treatments followed by aging at 900 °C for 10 h, on the microstructure of cast Ni-based IN100 superalloy were assessed. It has been found that, the alloy in the partial + full solution treated condition had the optimal combination of γ’ morphology, volume fraction and size. In this condition, the alloy possesses a cubic primary γ’with an average size of 470±10nm and 45% volume fraction. Discrete M23C6 and M6C carbides were formed at the grain boundaries and the morphology of the cubic MC carbide was changed to the spherical shape. In addition, the volume fraction of γ’/γ eutectic phase dropped to half of its value, compared to the as-cast alloy. During partial solution treatment followed by aging, discrete carbides were formed at the grain boundaries. This treatment without full solutioning was not an effective method to provide an optimal volume fraction and arrangement of γ’ and MC carbides morphology. Full solutioning alone, changed the cubic morphology of the primary γ’ and the blocky MC carbides to the spherical shape.

Effect of cooling rates from partial solution temperature and aging on γ′ precipitation in IN792 superalloy

One of the important factors affecting the microstructure of Ni base superalloys is the cooling rate from partial solution temperature; therefore, the prime purpose of the present research is to investigate the effects of this factor on some microstructural features of superalloy IN792. The results showed that by increasing the cooling rate from 5 to 160°C min−1 after partial solution treatment at 1120°C for 2 h, the size and volume fraction of primary γ′ decreased, but during the subsequent aging the volume fraction of secondary γ′ increased initially exponentially and later linearly. When high cooling rates >90°C min−1 were applied, the formation of secondary γ′ was totally suppressed. However, after aging at 845°C for a period of 24 h, the amount of secondary γ′ increased with a decelerating rate up to 60°C min−1 and then with a linear rate up to 160°C min−1. The later observation has not been reported elsewhere.

Ni基単結晶超合金の表面溶融部の再結晶挙動と機械的特性

Recrystallization behavior in postheat treatment of the surface-melted regions of Ni-base single crystal superalloy CMSX-4 have been investigated. In, addition, mechanical properties of surface-melted regions after postheat treatment were evaluated. The surface melting was conducted using 2kW diode laser. The microstructure and crystal orientation in the melted regions were analyzed by using optical microscopy, scanning electron microscopy (SEM) and electron backscattered pattern analysis (EBSP). The effect of postheat treatment condition on microstructure was investigated using the specimens whose surface-melted regions were single-crystallized. The surface-melted regions were recrystallized when the temperature of postheat treatment was higher than 1513K. On the basis of this result, the partial solution treatment condition was proposed as 1493K×3.6ks to maintain single crystal structure without recrystallization in the surface-melted regions. Furthermore, the mechanical properties of surface-melted regions after partial solution treatment and subsequent aging were investigated by the creep rupture test. The creep rupture strength of surface-melted specimens was comparable to those of the base metal. It could be concluded that the crystalline controlled repairing technique would be applicable to the repair of single crystal superalloy turbine blades.

Microstructural Development in Bearing Steel during Rolling Contact Fatigue

It is well known that microstructural changes occur in a steel bearing, when the bearing is operated under conditions involving high cyclic stresses. When combined with relatively high temperatures, such microstructural changes result in the flaking of the bearing raceway. In this paper, microstructural changes that occurred during rolling contact fatigue were investigated, and the relationship between these changes and fatigue life are discussed in association with the recrystallization behavior of martensite. Conventional bearing steel SUJ2 (SAE52100) was subjected to partial solution treatment at 1133K for 2.4ks followed by oil quenching. The quenched material with a martensitic structure was tempered at 443K for 7.2ks, and then subjected to rolling contact fatigue testing. The testing was performed at temperatures ranging from 373K to 443K and surface pressures of 4.6GPa or 5.5GPa. During testing at 373K, flaking occurred from the surface of the raceway due to non-metallic inclusion and without any marked microstructural changes. On the other hand, in the case of testing at 403K or more, flaking occurs after obvious microstructural changes. Firstly, dark etching constituent (DEC) formed around the area of maximum shear stress, which was followed by the formation of white etching constituent (WEC) within the DEC at 80 and 30 degrees to the rolling direction. TEM observations showed the change from martensite lath to dislocation cell structure within the DEC, and also the existence of fine ferrite grains of 20nm through 100nm within the WEC. Arrhenius plots for the fatigue life indicated that the activation energy of the fatigue process corresponded to that of carbon diffusion in bcc ferrite. These results suggest that rolling contact fatigue originated from the WEC is controlled by the diffusion of carbon in the ferrite matrix.

部分溶体化処理した12%Cr-0.3%C鋼の恒温変態挙動

部分溶体化処理した12%Cr-0.3%C鋼の恒温変態挙動

Microstructure Control for Toughening a High Carbon Martensitic Stainless Steel

A High carbon martensitic stainless steel (Fe-12%Cr-0.7%C) was subjected to isothermal aging after full solution treatment, and then solution-treated again in the two phase region of austenite and M23C6 carbide; partial solution (PS) treatment. Microstructural development during these heat treatment was investigated, and mechanical properties for the PS treated steel were evaluated by comparison with a referencial steel without isothermal aging. During the isothermal aging at 923K, the supercooled austenite undergoes eutectoid transformation and decomposes to ferrite and M23C6 carbide, and the carbide particles are finely dispersed within the matrix. Insoluble carbide particles retained during the following PS treatment contribute to suppressing austenite grain growth effectively through the grain boundary pinning effect. Ductile-brittle transition in the PS treated steel is characterized by a lower transition temperature and a higher upper-shelf energy compared with the referencial steel. This is due to the grain refining of prior austenite as well as the homogeneous dispersion of carbide particles through the PS treatment with isothermal aging.

Diffusion Bonding of CMSX-4 to UDIMET 720 Using PVD-Coated Interfaces and HIP

There is an increasing interest in development of manufacturing methods for Dual Property BLISKs, consisting of creep resistant airfoils and fatigue resistant disks bonded together by a durable joint. Optimum heat treatments are, however, very different for creep resistant single crystal CMSX-4 and fatigue resistant polycrystalline Udimet 720 selected in this study, but fortunately the first aging treatment for CMSX-4 (1140°C, 2–6 h, AC) is similar to the partial solution treatment of U 720 HS2 (115°C, 4h, OQ). Based on this, diffusion bonding was performed by HIP at 1120°C and 200 MPa argon pressure for 4 h, followed by cooling to 400°C. Subsequently, a shortened Udimet 720 HS2 two-step aging treatment was adopted by heating to 650°C for 6 h followed by cooling to 400°C, heating to 760°C for 2 h, and finally cooling to R.T. under remaining HIP pressure. Plasma etching followed by thin (80 nm) PVD coating with either nickel or titanium were used to clean and protect the polished surfaces before joining. The selection of coatings was governed by the possibility to reduce oxidized nickel by flushing with hydrogen at 330°C during evacuation of the HIP capsules, and by the large solubility of oxygen in titanium. Hot tensile testing was performed at 750°C on both joined and reference materials subjected to the modified heat treatment. Initially solution treated Udimet 720 and CMSX-4 comprised the reference materials. The testing showed that joints with Ti-PVD coatings were almost as strong as Udimet 720 (although with very limited elongation), while the joints with Ti-PVD coatings were weaker.

Effect of Partial Solution Treatment on Toughness of 12%Cr-0.3%C Steels.

In 12%Cr-0.3%C martensitic steels, the effect of partial solution (PS) treatment in the (γ+carbide) two phase region was investigated in terms of the relation between microstructure and impact toughness. PS treatment is very effective for refining γ grains, because insoluble carbide particles works as obstacles to grain growth. The γ grain refining results in a significant lowering of Ductile-Brittle Transition Temperature (DBTT). This is mainly due to the reduction of grain boundary stress concentration which is caused by pile-up of dislocations to grain boundary. Besides, PS treatment contributes to homogeneous dispersion of carbide particles: In specimens with full solution (FS) treatment, carbide particles precipitate along lath boundary of martensite during tempering, and this causes early crack initiation in impact testing. On the other hand, in specimens with PS treatment, carbides preferentially precipitate on insoluble carbide particles which have homogeneously dispersed within the martensite matrix. This results in an increase of upper shelf energy in impact testing. It was also found that PS treatment suppresses precipitation of grain boundary carbide which occurs during slow cooling from γ phase region, because the insoluble carbide particles provide preferential precipitation site within γ grains. As a result, DBTT is not dependent on cooling rate in PS treated specimens, although it is greatly raised in FS treated specimens with slow cooling because of the grain boundary precipitation of carbide.

Effect of two phase warm rolling on aging behavior and mechanical properties of Ti–15Mo–5Zr–3Al alloy

The isothermal aging behavior and change in tensile and mechanical properties of Ti–15Mo–5Zr–3Al alloy have been studied after three different pre-treatments, i.e. PR (rolling at partial solution temperature), PRP (PR+partial solution treatment) and C (complete solution treatment) by hardness and resistivity measurements, tensile and Charpy impact tests. After the PR and the PRP treatment, almost all of the plate-like primary α particles which were left in as-received specimens became granular, furthermore age-hardening was observed in that α+ β bi-phased specimens. Plate-like α is always formed by the aging. Sub-grain size of β phase in the PR and the PRP specimens is 1–2 μm, and no grain growth is observed during the aging. Tensile properties after 673 K-600 ks or 773 K-60 ks aging are improved by the PR or the PRP than the C pre-treatment. The fracture surface of the aged and tensile tested PR and PRP specimens shows dimples, whereas that of the C specimen shows flat surface, area fraction of 40–50%, due to grain boundary fracture. Deflection–load curve shows practically no change in its shape with variation in the pre-treatment, however maximum load is highest in the PRP specimen and decreases in order of the PR and the C. Total absorbed energy is also larger in the PR and the PRP specimens than in the C specimen in age-hardened state. The best strength–ductility balance is attained by the PRP specimen aged at 773 K for 60 ks.

12%Cr鋼のマルテンサイト変態に及ぼす部分溶体化処理の影響

12%Cr鋼のマルテンサイト変態に及ぼす部分溶体化処理の影響

Heat treatment of cast Co-Cr-Mo for orthopaedic implant use

Cast Co-Cr-Mo was heat treated to determine whether a significant improvement in the alloy's mechanical properties could be obtained without loss of corrosion resistance. Tensile, corrosion fatigue, and corrosion tests were carried out using individually cast test pieces. The effect on the tensile properties of solution treatment at 1240° C and of ageing at 720° C was determined for a large range of heat treatment times. In addition the effect of other heat treatments described in the literature was determined. Partial solution treatment gave the greatest improvement in corrosion fatigue behaviour. None of the heat treatments significantly affected the corrosion resistance. In spite of the improvements obtained, it was concluded that for orthopaedic implant applications requiring high corrosion fatigue strength, cast Co-Cr-Mo was less suitable than other currently available products.