Age hardening and mechanical properties of a 2400 MPa grade cobalt-free maraging steel

The age hardening kinetics in the temperature range of 713 to 813 K of a 2400 MPa grade cobalt-free maraging steel (Fe-(18.8 ∼ 19.1) pct Ni-(4.4 ∼ 5.4) pct Mo-2.6 pct Ti, wt pct) has been studied. Study of microstructure and mechanical properties showed that a high number of Ni₃Ti and Fe₂(Mo,Ti) precipitates were formed during the ageing process, which resulted in high strength and relatively low fracture toughness. Ni₃Ti was the main precipitation phase. Fractography has shown ductile failure of tensile and fracture toughness specimens. Thermodynamic calculations showed that the equilibrium phases are Ni₃Ti, Fe₂(Mo,Ti), ferrite, and austenite.     

Thermal embrittlement of 18 Ni(350) maraging steel

The embrittlement of as-solutionized 18 Ni(350) Maraging steel was monitored as a function of heat treatment variables by means of Charpy impact tests. The processing parameters of interest were annealing temperatures in the range of 1900° to 2400°F, intermediate holding temperatures in the range of 1300° to 1800°F, and the quenching rate. The changes in fracture mode with heat treatment were characterized by replica and scanning electron microscopy. The severity of thermal embrittlement increases with decreasing cooling rate from the annealing treatment upon direct quenching to room temperature. Intermediate isothermal holding, particularly at 1500° to 1600°F, further accentuates the embrittlement. A large grain size is beneficial to the toughness when rapid direct quenches from the annealing range are imposed but is detrimental upon air cooling or intermediate holding. The major loss in toughness may be associated with the diffusion of interstitial impurity atoms (C+N) to the austenite grain boundaries during cooling or intermediate isothermal holding below 2000°F. An advanced stage of the embrittlement is characterized by the discrete precipitation of Ti(C,N) platelets on these boundaries. Thermal embrittlement is accompanied by change in fracture mode from transgranular dimpled rupture to intergranular quasi-cleavage.     

Effect of thermal cycling on the mechanical properties of 350-grade maraging steel

The effects of retained austenite produced by thermal cycling on the mechanical properties of a precipitation-hardened 350-grade commercial maraging steel were examined. The presence of retained austenite caused decreases in the yield strength (YS) and ultimate tensile strength (UTS) and effected a significant increase in the tensile ductility. Increased impact toughness was also produced by this treatment. The mechanical stability of retained austenite was evaluated by tension and impact tests at subambient temperatures. A deformation-induced transformation of the austenite was manifested as load drops on the load-elongation plots at subzero temperatures. This transformation imparts excellent low-temperature ductility to the material. A wide range of strength, ductility, and toughness can be obtained by subjecting the steel to thermal cycling before the precipitation-hardening treatment.     

The strength and fracture toughness of 18 Ni (350) maraging steel

The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in Ni₃Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe₂Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite.     

Precipitation hardening

The topic of precipitation hardening is critically reviewed, emphasizing the influence of precipitates on the CRSS or yield strength of aged alloys. Recent progress in understanding the statistics of dislocation-precipitate interactions is highlighted. It is shown that Pythagorean superposition for strengthening by random mixtures of localized obstacles of different strengths is rigorously obeyed in the limit of very weak obstacles; this had been known previously as a result of computer simulation experiments. Some experimental data are discussed in light of this prediction. All of the currently viable mechanisms of precipitation hardening are reviewed. It is demonstrated that all versions of the theory of coherency hardening are woefully inadequate, while the theory of order hardening is capable of accurately predicting the contribution of γ′ precipitates to the CRSS of aged Ni-Al alloys. It is also convincingly shown that a new theory based on computer simulation experiments of the motion of dislocations through arrays of obstacles having a finite range of interaction cannot explain these same data, and is of doubtful validity in other instances for which its success has been proclaimed. A new theory of hardening by spinodal decomposition is proposed. It is based on the statistics of interaction between dislocations and diffuse attractive obstacles, and is shown to be in very good quantitative agreement with much of the limited data available. Some of the problems that remain to be addressed and solved are discussed. This paper is based on a presentation made at the symposium “50th Anniversary of the Introduction of Dislocations” held at the fall meeting of the TMS-AIME in Detroit, Michigan in October 1984 under the TMS-AIME Mechanical Metallurgy and Physical Metallurgy Committees.     

Microstructure and mechanical properties of a 2000 MPa grade co-free maraging steel

The precipitation kinetics in the aging temperature range of 713 to 813 K in a 2000 MPa grade Co-free maraging steel (Fe-18.9 pct Ni-4.1 pct Mo-1.9 pct Ti, mass pct) has been studied. Study on microstructure and mechanical properties showed that a great deal of Ni₃Ti and a type of unknown spheroidal precipitates both with average diameter of 2 to 3 nm are formed in the early aging stage at 713 K, which results in a high strength and a relatively low fracture toughness. Ni₃Ti precipitates grow into needle or rod shape and become the main precipitation as the aging time is prolonged. Strength increases and fracture toughness (K _IC ) decreases with growth of the precipitates. The ultra-high strength of the maraging steel subjected to long-time aging at 713 K is attributed to the high resistance to coarsening of the precipitates. The strengthening in the underaged condition at 713 K is a combination of dislocations cutting through precipitates and the Orowan mechanisms. Aged at 813 K, the size of Ni₃Ti precipitates is seriously nonuniform at the early stage and a small amount of interlath reverted austenite is formed. Thereafter, Ni₃Ti precipitates coarsen sharply accompanied with the embrittlement. Intralath reverted austenite appears subsequently. In the later stage of aging, the coarsened Ni₃Ti precipitates dissolve into the striplike intralath reverted austenite that is disorderly embedded in the matrix. All of these result in a low strength and low fracture toughness under overaging condition. Analysis shows that the formation of reverted austenite contains the diffusion and Kudjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) shear mechanisms.     

Improved fatigue resistance of 18Ni (350) maraging steel through thermomechanical treatments

The influence of thermomechanical treatments, ausforming and marforming, on the fatigue resistance of 18 Ni (350) maraging steel has been examined. Although the low cycle fatigue resistance of this material is essentially unaffected by these treatments, an increase of 30 pct in the low-stress, high-cycle fatigue resistance can be achieved. This increase can be explained by considering the influence of processing on the resulting precipitate and dislocation substructures. Differences in texture, residual stress level and inclusion morphology have no effect on the improved fatigue resistance.     

非钙处理对高等级齿轮钢夹杂物的影响

 为解决水口堵塞、B类夹杂物超标及控制Ds类夹杂物,通过热力学计算和工业试验研究了SCr420H齿轮钢钙处理与非钙处理对中间包夹杂物的影响。研究发现,非钙处理炉次中间包夹杂物数量低于钙处理炉次。非钙处理炉次中间包夹杂物主要为棱角状镁铝尖晶石;钙处理炉次夹杂物主要为Al₂O₃-CaO-MgO与CaS复合球状夹杂物。优化工艺后非钙处理炉次中间包高熔点尖晶石类夹杂物数量大幅降低,水口堵塞及B类夹杂物问题得到解决。非钙处理圆钢夹杂物评级结果为,A类不大于0.5,B类不大于0.5级,C类为0,D(细)类不大于1.0级,D(粗)类不大于0.5级,Ds类不大于1.0级。     

Increased fracture toughness in a 300 grade maraging steel as a result of thermal cycling

Systematic changes in the fracture toughness of a 300 grade commercial maraging steel were obtained using a non-standard heat-treating process. Microstructures consisting of variable amounts of retained austenite in an aged martensitic matrix were produced. A mathematical model is presented relating these toughness values to the properties of the individual constituents. Increases in fracture toughness resulting from the non-standard heat-treatment were attributed to the nature of the distribution of the tough phase (retained austenite) in a brittle matrix of precipitation hardened martensite. In some cases, a strain-induced transformation to martensite was observed which greatly added to the toughness. Some improvements in fatigue crack propagation characteristics also resulted from this heat treatment. Formerly Postdoctoral Fellow, Department of Materials Science and Metallurgical Engineering, University of Cincinnati     

Precipitation hardening in Mg-Ca-Zn alloys

Based on thermodynamic calculations, four alloys in the system Mg-Ca-Zn produced by gravity die casting were solution treated and aged at 175 °C for different times. The scanning electron microscopy and X-ray diffraction analyses of the solutionized alloys were in good agreement with the thermodynamic calculations. In these Mg-Ca-Zn alloys, the hardness increases with aging time up to 8 hours followed by typical overaging, during which the hardness decreases. According to the calculations and scanning transmission electron microscopy analyses, the change in the hardness can be correlated to the precipitation of Ca₂Mg₆Zn₃.     

Precipitation hardening in Cu-Zn-Be alloys

Brasses containing 20 and 30 pct Zn and up to 1.15 pct Be have been examined to provide tensile test data on the age-hardening response of these ternary alloys. It was found that yield strengths of 1100 Mpa were attainable with cold working prior to aging at 300°C. The mode of precipitation, as determined by electron micros copy and electron diffraction, agrees substantially with recent work on the binary Cu-Be system. Precipitation commences as GP zones on the (001)_α planes. The observed precipitate phase, after aging times of up to seven days at 400°C, is CuBe with an orientation relationship of the type (301)_ppt ∥(113)_α [010]_ppt ∥[110]_α.     

Precipitation of TiC in thermally embrittled maraging steels

It is shown that maraging steels can be embrittled by the precipitation of TiC during slow cooling and/or intermediate annealing in the austenite temperature range. An important aspect in this embrittlement is the occurrence of lamellar precipitation of TiC at the austenite grain boundaries, generating a cellular structure of large fern leaf-like carbides. Within the austenite grains a nonuniform distribution of irregularly plate-shaped TiC particles are formed with (100) austenite habit orientation. Quenching to martensite, prior to any intermediate anneals, changes the carbide distribution upon subsequent annealing treatments into a fine dispersion of TiC particles. The embrittlement resulting from the various isothermal annealing treatments in the austenite temperature region could all be directly related to the carbide distribution in the prior austenite grain boundary region.     

Precipitation hardening in Cu-Zn-Be alloys

Brasses containing 20 and 30 pct Zn and up to 1.15 pct Be have been examined to provide tensile test data on the age-hardening response of these ternary alloys. It was found that yield strengths of 1100 Mpa were attainable with cold working prior to aging at 300°C. The mode of precipitation, as determined by electron micros copy and electron diffraction, agrees substantially with recent work on the binary Cu-Be system. Precipitation commences as GP zones on the (001)_α planes. The observed precipitate phase, after aging times of up to seven days at 400°C, is CuBe with an orientation relationship of the type (301)_ppt ∥(113)_α [010]_ppt ∥[110]_α.     

Precipitation of TiC in thermally embrittled maraging steels

It is shown that maraging steels can be embrittled by the precipitation of TiC during slow cooling and/or intermediate annealing in the austenite temperature range. An important aspect in this embrittlement is the occurrence of lamellar precipitation of TiC at the austenite grain boundaries, generating a cellular structure of large fern leaf-like carbides. Within the austenite grains a nonuniform distribution of irregularly plate-shaped TiC particles are formed with (100) austenite habit orientation. Quenching to martensite, prior to any intermediate anneals, changes the carbide distribution upon subsequent annealing treatments into a fine dispersion of TiC particles. The embrittlement resulting from the various isothermal annealing treatments in the austenite temperature region could all be directly related to the carbide distribution in the prior austenite grain boundary region.     

Precipitation hardening of aluminum alloys

The author’s charge was to discuss recent trends in research and development on precipitation hardened aluminum alloys and to indicate where research is needed. This will be done for three areas: fatigue, properties of grain boundaries and interfaces, and stability of precipitates at elevated temperatures. Present strong precipitation hardened aluminum alloys do not have high endurance limits. One problem is that the small GP zones are cut by the dislocations giving rise to highly localized deformation which aids fatigue crack initiation. A duplex structure with relatively large uniformly spaced precipitates to give more homogeneous deformation plus small precipitates to give high yield strength is a promising approach. The structures of precipitation hardened aluminum base alloys are essentially controlled by the stabilities of the various precipitates and the interfacial energies. Precipitates with high interfacial energies tend to precipitate preferentially at grain boundaries giving embrittlement. Low interfacial energy means easy nucleation, a uniform precipitate distribution, and resistance to coarsening at elevated temperatures. For elevated temperature use, the precipitate must be stable at elevated temperatures. Precipitation hardened aluminum alloys do not have good elevated temperature properties because the hardening precipitates normally used, GP zones, are not stable at elevated temperatures. Thus a low interfacial energy, ductile precipitate, which is stable at elevated temperatures, is needed for aluminum. Possibilities for achieving such precipitates will be discussed.     

Fatigue behavior of maraging steel 300

The cyclic stress-strain curves, the low cycle and high cycle fatigue lives and the fatigue crack growth rates of annealed (1 h 820°C) and aged (3 h 480°C) maraging steel 300 were determined. Incremental step testing and stable hysteresis loop tip measurements were used to determine the cyclic σ-ε curves. Both annealed and aged maraging steels were found to cyclically soften at room temperature over a plastic strain range from 0.1 to 20 pct. The S-N curves were determined from 10 to 10⁷ cycles to failure by plastic strain controlled low cycle fatigue tests performed in air and load controlled high cycle fatigue tests performed in dry argon. The test results compared very well with the theoretical lifetime predictions derived from Tomkins’ theory. Fatigue crack growth rates were measured in air and dry argon for the annealed and aged alloys. Crack growth rates of annealed maraging steel were found to be equal to those of aged maraging steel at rates between 10^-7 and 10^-5 in./cycle. A significant difference in crack growth rates in the two environments was found at low stress intensity factor ranges, indicating a high susceptibility to corrosion fatigue in the presence of water vapor. The mechanisms of cyclic softening in the two alloys are discussed in terms of dislocations rearrangement in the annealed alloy and dislocation-precipitate interactions in the aged alloy.     

Precipitation hardening of HSLA steels

A process model to describe the strength contribution from precipitation hardening during coiling after hot rolling has been developed for V and Nb HSLA steels. Experimental measurements of ageing behaviour on the V steel were conducted on coil material which was received in an underaged condition. The size and composition of precipitates was examined on replicas using scanning transmission electron microscopy. The precipitates were observed to be V rich and a substantial increase in precipitate size occurred as a function of ageing time. The modelling approach developed by Shercliff and Ashby for aluminium alloys was extended to microalloyed steels. The model assumes particle coarsening is the rate controlling process and that the precipitates are initially sheared by dislocations with a transition to non-shearable precipitates at peak strength. After calibration of the model, good agreement was observed between the model predictions and experimental data. A model for ageing in a Nb HSLA steel was also developed using literature values for the ageing behaviour.     

Precipitation hardening of aluminum alloys

The author’s charge was to discuss recent trends in research and development on precipitation hardened aluminum alloys and to indicate where research is needed. This will be done for three areas: fatigue, properties of grain boundaries and interfaces, and stability of precipitates at elevated temperatures. Present strong precipitation hardened aluminum alloys do not have high endurance limits. One problem is that the small GP zones are cut by the dislocations giving rise to highly localized deformation which aids fatigue crack initiation. A duplex structure with relatively large uniformly spaced precipitates to give more homogeneous deformation plus small precipitates to give high yield strength is a promising approach. The structures of precipitation hardened aluminum base alloys are essentially controlled by the stabilities of the various precipitates and the interfacial energies. Precipitates with high interfacial energies tend to precipitate preferentially at grain boundaries giving embrittlement. Low interfacial energy means easy nucleation, a uniform precipitate distribution, and resistance to coarsening at elevated temperatures. For elevated temperature use, the precipitate must be stable at elevated temperatures. Precipitation hardened aluminum alloys do not have good elevated temperature properties because the hardening precipitates normally used, GP zones, are not stable at elevated temperatures. Thus a low interfacial energy, ductile precipitate, which is stable at elevated temperatures, is needed for aluminum. Possibilities for achieving such precipitates will be discussed. This paper is based on an invited presentation made at a symposium on “Advances in the Physical Metallurgy of Aluminum Alloys” held at the Spring Meeting of TMS-IMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD.