Magnetic properties of maraging steel in relation to deformation and structural phase transformations

Magnetic properties in annealed and cold rolled conditions have been investigated for maraging steel grade 18%Ni-2400. The austenite content, coercive field, saturation magnetisation and remanence were determined after ageing for 1 h in the temperature range from 400 to 800°C. The results show that the degree of deformation imparted to martensite influences both the amount of reverted austenite and the magnetic properties obtained following ageing. Transmission electron microscopy was carried out in order to investigate the structural changes taking place during reversion of austenite.     

Mechanical properties of new low‐nickel cobalt‐free maraging steels

To study the mechanical properties of newly developed low‐nickel cobalt‐free maraging steels, six laboratory series with varied Ti, Mo, and Cr mass contents (0.003‐ 1.65, 0.0074‐ 5 and 0.004‐ 5 %) were studied. The study showed that increasing Ti and Mo contents improve the tensile strength and yield strength without affecting ductility markedly. The chromium alloyed grades M2, M6, M8 and M12 exhibit properties comparable to those of T 250 grade. To investigate the effects of heat treatment temperature on the mechanical properties, the solution treatment was carried out at temperatures varying between 820 and 1100 °C and holding times of 15 min at different cooling rates. The ageing temperature varied from 400 to 500 °C at ageing times of 60, 120 and 240 min, respectively. The ageing response of hardness as a function of temperature at different soaking conditions was determined to optimise the heat treatment conditions. The mechanical properties of the different steels after solution treatment and ageing at optimum temperature and time were determined.     

Precipitation reactions and strengthening behavior in 18 Wt Pct nickel maraging steels

The crystallography, structure, and composition of the strengthening precipitates in maraging steels C-250 and T-250 have been studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns. The kinetics of precipitation were studied by electrical resistivity and microhardness measurements and could be described adequately by the Johnson-Mehl-Avarami equation, with precipitate nucleation occurring on dislocations and growth proceeding by a mechanism in which the dislocations serve as collector lines for solute from the matrix along which pipe diffusion occurs. The strengthening of the Co-free, higher Ti T-250 steel is caused by a refined distribution of Ni₃Ti precipitates. High strength is maintained at longer times from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. In the case of the Co-containing, lower Ti C-250 steel, strengthening results from the combined presence of Ni₃Ti (initially) and Fe₂Mo precipitates (at longer times). Loss of strength at longer times is associated, in part, with overaging and mainly from the larger volume fraction of reverted austenite. The resistance to austenite reversion is dependent on the manner in which the relative nickel content of the martensite matrix is affected by the precipitating phases, and the difference in the reversion tendency between the two steels can be explained on this basis. Formerly with the University of Illinois, Formerly with the University of Illinois     

The mechanical stability of austenite in maraging steels

Two different types of retained austenite were encountered in maraging steels: one obtained by the usual overaging/reversion process, and was found to be mechanically unstable at room temperature; the other, obtained in a sequence of isothermal heat treatments leading to the formation of microsegregational content zones of molybdenum and cobalt in the lath-martensite, and was found to be mechanically stable at room temperature (RT). In the unstable case the austenite transformed to martensite upon cold working at RT. In the stable case, as was shown by careful Mössbauer-effect spectroscopy and X-ray diffraction studies, the amount of retained austenite was not affected by the cold-working at RT, whereas some amount of the martensite was transformed into a ferromagnetic-type of austenite. Complementary studies by electron diffraction have shown that both Kordjumov-Sachs and Nishiyama crystallographic orientation relationships may exist between austenite and martensite, depending on the local molybdenum and cobalt segregational contents in the lath martensite. The appearance of ferromagnetic austenite, as well as other segregational effects observed by the Mössbauer-effect spectroscopy are discussed.     

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 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.     

Use of corrosion-resistant maraging steels in long-lived thread pieces

The effect of isothermal holdings at 350, 500, 580, 660, and 780°C during heating to various sustenitizing temperatures on the grain size, aging processes, structural and mechanical properties, and the stress-strain curves of 03Kh11N10M2T-VD and 03Kh11N8M2F-VD maraging steels with 0.002% B is studied. X-ray diffraction analysis and metallographic examination are performed, and the corrosion and fatigue characteristics of these steels are determined. At the aged state with the maximum strength, the steels exhibit no strain-hardening ability upon tension and retain a high local plastic deformation during necking (ψ ≤ 60%). Preliminary thermal-cycling treatment at 500–800°C causes grain refinement and increases the plastic properties of the steels (the uniform elongation increases to 20%). Isothermal holding during heating to the austenitizing temperature affects an elastoplastic transition at a low tensile strain.     

The morphology and substructure of martensite in maraging steels

Thin foil transmission electron microscopy, X-ray diffraction and dilatometric techniques have been used to study the martensitic γ → α transformation in three steels with nominal contents of 8 pct nickel and 0.2 pct beryllium and chromium contents of 12, 14 and 16 pct. In each case the martensite formed as laths with a habit plane close to {225}_γ. With increasing chromium content and increasing cooling rate greater numbers of the laths were observed to be internally twinned. Detailed analysis of the martensitic transformation suggested that the internally twinned laths are formed by a sequence of γ→ ε or faulted γ→ ά. The orientation relationships between the three phases γ, ε and α, determined from selected area diffraction analysis, corresponded to Kurdjumov-Sachs.     

Hydrogen enhanced crack growth in 18 Ni maraging steels

The kinetics of sustained-load subcritical crack growth for 18 Ni maraging steels in high purity hydrogen are examined using crack-tip stress intensity,K, as a measure of crack driving force. Crack growth rate as a function of stress intensity exhibited a clearly definedK-independent stage (Stage II). Crack growth rates in an 18 Ni (250) maraging steel are examined for temperatures from -60°C to 100°C. A critical temperature was observed above which crack growth rates became diminishingly small. At lower temperatures the activation energy for Stage II crack growth was found to be 16.7 ± 3.3 kJ/mole. Temperature and hydrogen partial pressure are shown to interact in a complex manner to determine the apparentK _th and the crack growth behavior. Comparison of results on ‘250’ and ‘300’ grades of 18 Ni maraging steel indicate a significant influence of alloy composition and/or strength level on the crack growth behavior. These phenomenological observations are discussed in terms of possible underlying controlling processes. Formerly a Graduate Student and Research Assistant. Based on a thesis submitted in partial fulfillment of requirements for the M.S. degree in Metallurgy and Materials Science, Lehigh University, Bethlehem, PA.     

Mechanical properties improvement of PM 400 series stainless steels via nickel addition

PM 409L and 434L grades of stainless steels have recently been selected at the optimum materials for automotive exhaust system applications, such as flanges and HEGO bosses. These applications are much more demanding than the most current PM stainless steel applications in terms of mechanical properties, weldability, and resistance to corrosion and thermal fatigue. Also important for these applications are elevated temperature mechanical strength, impact strength and resistance to stress relaxation at elevated temperatures. Significant improvements in both the room and elevated temperature mechanical stregths of these PM materials can be achieved by alloying with small amounts of nickel. The benefits of this approach are found to be significant in the case of 409L and moderate in the case of 434L.     

The effects of thermal treatment on the austenitic grain size and mechanical properties of 18 Pct Ni maraging steels

A technique has been developed for controlling the austenitic grain size of 18 pct Ni maraging steels by thermal treatment alone. This treatment has been applied to two different grades, 250 and 300, of maraging steel, and a large grain size, ASTM 2, was reduced to ASTM 7 in both cases. The process of grain size refinement requires thermal cycling from a temperature belowM _f to a temperature considerably above the austenitizing temperature. The minimum austenitic grain size attainable depends on the prior strain in the material as well as the thermal treatment. While significant grain size refinement can be attained by one cycle to the proper temperature, the attainment of the minimum uniform grain size requires several cycles. The effects of austenitic grain size on tensile properties have been investigated both at room temperature and at elevated temperatures. The prior austenitic grain size has a small but measurable effect on the mechanical behavior of aged material at room temperature. The austenitic grain size has a significant effect on the ultimate tensile strength at 1600°F.     

Strain induced martensite formation in PM nickel steels

Abstract

Powder metallurgy (PM) nickel steels are often selected because they have high strength, high impact resistance and good abrasion resistance. The microstructure of slowly cooled PM nickel steels typically contains pearlite, bainite, martensite and a fair volume fraction of retained austenite. Since volume diffusion is really low at conventional sintering temperatures [1120–1150°C (≈2050–2100°F)], nickel rich areas are usually found where prior admixed nickel particles were located, i.e. at the surface of iron particles and in sintering necks. Therefore, there is a discrepancy between the rather low mechanical properties of austenite and the high strength of PM nickel steel. Hence, the hypotheses that stress induced or strain induced martensite formation takes place during mechanical testing have been investigated. Results show that martensite forms during mechanical testing, and its final volume fraction is proportional to nickel content.     

氮含量对无镍奥氏体不锈钢理化性能的影响

高氮无镍奥氏体不锈钢在多个领域有着广泛的应用前景,然而氮质量分数对其物理性能和化学性能的影响规律尚不十分清晰。设计并冶炼了氮质量分数为0.02%~1.20%的无镍奥氏体不锈钢,对不同氮质量分数钢的晶格常数、线膨胀系数、电阻率、耐汗液腐蚀性能、自腐蚀电位进行了检测和分析。结果表明,氮质量分数的增加使材料的晶格常数随着线性增大,获得了氮质量分数与晶格常数的定量关系。氮质量分数越高,材料的膨胀系数和电阻率越大。不同氮质量分数试验钢经720 h人工汗液腐蚀后,氮质量分数最低(0.02%)和最高(1.20%)时,试验钢严重腐蚀,而其他钢未被腐蚀。试验钢的自腐蚀电位首先随着氮质量分数的增加而升高,峰值出现在氮质量分数为1.05%时,氮质量分数为1.20%的钢其自腐蚀电位下降至低于氮质量分数为0.60%钢的水平。     

Phase chemistry and precipitation reactions in maraging steels: Part III. Model alloys

This article describes studies of phase transformations during aging in a variety of model maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Thermochemical calculation was also used during the course of the work. The composition and morphology of precipitates were compared in several maraging systems aged at different temperatures for different times to investigate the aging sequence. The APFIM results are compared with studies by other workers using different experimental techniques. In Fe-Ni(-Co)-Mo model alloys, ω phase and Fe₇Mo₆ μ phase have been found to contribute to age hardening at different stages of aging; no evidence was found for the existence of Mo-rich clusters in the as-quenched Fe-Ni-Co-Mo alloy. In a high-Si Cr-containing steel, Ti₆Si₇Ni₁₆ G phase and Ni₃Ti have been found to contribute to age hardening; reverted austenite was found after aging for 5 hours at 520 °C. In a Mn-containing steel, Fe₂Mo Laves phase and a structurally uncertain phase with a composition of Fe₄₅Mn₃₂Co₅Mo₁₉ have been found to contribute to age hardening. Formerly Graduate Student with the Department of Materials, Oxford University.     

Structure and properties of a microduplex maraging steel

A simple two-step thermal processing technique was devised to impart a microduplex structure in a high strength 250 grade commercial maraging steel. A martensite grain size of approximately 1μm was obtained with interspersed islands of retained austenite whose volume fraction and mechanical stability could be controlled by varying the thermal processing conditions. The microstructure and mechanical properties of the microduplex structure were compared to those of the alloy in the maraged, martensitic condition. Due to the presence of the austenite phase, the microduplex structure showed a much smaller temperature and strain rate dependence of deformation than the martensitic structure. A remarkable increase in uniform elongation was observed below theM _d temperature of retained austenite. The microduplex structure did not show any significant advantage in fracture toughness over the martensitic structure when compared at similar strength levels. By suitably adjusting austenitic stability a deformation-induced phase transformation (TRIP) of the retained austenite in the microduplex structure could be made to occur; however, the transformation did not lead to any evident increase in toughness. The microduplex structure exhibited a slight improvement in fracture toughness at high strain rate in contrast to the martensitic structure in which the rate effect significantly reduced the toughness.     

Structure and properties of a microduplex maraging steel

A simple two-step thermal processing technique was devised to impart a microduplex structure in a high strength 250 grade commercial maraging steel. A martensite grain size of approximately 1 μm was obtained with interspersed islands of retained austenite whose volume fraction and mechanical stability could be controlled by varying the thermal processing conditions. The microstructure and mechanical properties of the microduplex structure were compared to those of the alloy in the maraged, martensitic condition. Due to the presence of the austenite phase, the microduplex structure showed a much smaller temperature and strain rate dependence of deformation than the martensitic structure. A remarkable increase in uniform elongation was observed below theM _d temperature of retained austenite. The microduplex structure did not show any significant advantage in fracture toughness over the martensitic structure when compared at similar strength levels. By suitably adjusting austenitic stability a deformation-induced phase transformation (TRIP) of the retained austenite in the microduplex structure could be made to occur; however, the transformation did not lead to any evident increase in toughness. The micro-duplex structure exhibited a slight improvement in fracture toughness at high strain rate in contrast to the martensitic structure in which the rate effect significantly reduced the toughness.     

Phase chemistry and precipitation reactions in maraging steels: Part IV. Discussion and conclusions

This article summarizes our studies of phase chemistry and precipitation reactions in a variety of maraging steels. The roles of different phases and alloying elements are investigated by comparing the behavior of different steels. The phases considered are Ni₃Ti, Fe₇Mo₆ μ phase, Fe₂Mo Laves phase, ω phase, Ti₆Si₇Ni₁₆ G phase, “Z phase,” austenite, and α matrix. The alloying elements discussed are Ti, AI, Mo, Si, Mn, Ni, Cr, and Co. By comparing the aging behavior of both commercial steels and model alloys, a major role of Co is confirmed to be the lowering of the matrix solubility of Mo. Of the two main hardening elements in maraging steels (namely, Ti and Mo), Ti is much more active than Mo in the very early stage of precipitation. The main Mo-rich precipitate found in this work was Fe₇Mo₆μ phase instead of Laves phase. The precipitation of Mo is modified by the presence of Ti. ω phase appears only in Ti-free alloys, especially when aged at a low temperature. The quantity of Ni-containing precipitates and the presence of Cr in the steels change the austenite reversion behavior. Other phases, such as G phase and “Z phase,” contribute to age hardening in different types of maraging alloys. Formerly Graduate Student with the Department of Materials, Oxford University.