Optimization of process parameters for the manufacturing of rocket casings: A study using processing maps

Maraging steels possess ultrahigh strength combined with ductility and toughness and could be easily fabricated and heat-treated. Bulk metalworking of maraging steels is an important step in the component manufacture. To optimize the hot-working parameters (temperature and strain rate) for the ring rolling process of maraging steel used for the manufacture of rocket casings, a systematic study was conducted to characterize the hot working behavior by developing processing maps for γ-iron and an indigenous 250 grade maraging steel. The hot deformation behavior of binary alloys of iron with Ni, Co, and Mo, which are major constituents of maraging steel, is also studied. Results from the investigation suggest that all the materials tested exhibit a domain of dynamic recrystallization (DRX). From the instability maps, it was revealed that strain rates above 10 s⁻¹ are not suitable for hot working of these materials. An important result from the stress-strain behavior is that while Co strengthens γ-iron, Ni and Mo cause flow softening. Temperatures around 1125 °C and strain rate range between 0.001 and 0.1 s⁻¹ are suitable for the hot working of maraging steel in the DRX domain. Also, higher strain rates may be used in the meta-dynamic recrystallization domain above 1075 °C for high strain rate applications such as ring rolling. The microstructural mechanisms identified from the processing maps along with grain size analyses and hot ductility measurements could be used to design hot-working schedules for maraging steel.     

Stress-induced failure of high-strength steels in environments containing hydrogen sulphide

Abstract

The pre-cracked cantilever beam test is a sensitive means of evaluating the relative susceptibilities of different steels to stress-Induced failure in H₂S-containing environments. This, test indicated that a 12Ni 5Cr.3Mo maragmg stee1 is considerably more resistant to stress-induced crackining than the low-alloy steels 0Ni 1Cr 2Mo and 2Ni 1Cr 3Mo. For low-alloy steels with no applied potential the failure appears to be caused predominantly by hydrogen embrittlement. Impressed anodic or cathodic potentials have no effect on the time to Failure of low-alloy steels where as a zone of immunity exists for maraging steel with Impressed potentials Within the approximate range ?0·6 to ?1·0 V. This suggests that the maraging steel is less susceptible to hydrogen embrittlement than the quenched and tempered low-alloy steels. Permeability studies indicate that stress-induced failure is related to the ability of the steel to transmit hydrogen and to the nature of hydrogen traps in the steel.     

Einfluß von Molybdän und Aushärtungsgrad auf die Korrosionseigenschaften chromlegierter martensitaushärtender Stähle

Influence of molybdenum and degree of hardening on the corrosion properties of chromium alloyed maraging steels The influence of intermetallic precipitations on the corrosion resistance of maraging chromium alloyed steels with various molybdenum contents has been studied by tracing potentiokinetic and potentiostatic current density/potential curves. It has been shown that the passivation current density and the passive current density are measures of the corrosion resistance. Hardening times yielding maximum strength result in an increased dissolution current density; in this context increased molybdenum contents displace the increase of the current density toward longer times. Corrosion tests in artificial seawater distinctly show the positive influence of molybdenum which displaces the rapture potential toward more nobel values.     

残留/逆转变奥氏体对改善高强度不锈钢-196℃超低温冲击性能的影响

采用力学性能测试、SEM和XRD等手段研究了淬火+低温回火处理的0Cr16Ni6高强度不锈钢和过时效处理的00Cr11Ni11MoTi马氏体时效不锈钢,并分析了残留/逆转变奥氏体对试验钢超低温缺口抗拉强度和冲击性能的影响。结果表明,在两种试验钢室温强韧性相近的情况下,0Cr16Ni6钢在超低温下(-196 ℃)的缺口抗拉强度和冲击性能显著优于00Cr11Ni11MoTi钢。根据冲击试样远离断口和断口附近马氏体/奥氏体衍射峰的相对强度分别定量计算的残留/逆转变奥氏体含量,发现在裂纹形成和扩展过程中0Cr16Ni6钢有接近90%的残留奥氏体通过应变诱发相变生成马氏体,显著改善了超低温韧性;而过时效00Cr11Ni11MoTi钢形成的逆转变奥氏体具有较高的稳定性,难以发生应变诱发马氏体相变,改善超低温韧性作用程度有限。     

Developments and projected trends in maraging steels

The original development of maraging steels is reviewed, with emphasis on the basic inventive concepts involved. These concepts are related to the advantages of maraging steels. In addition to attractive combinations of strength and toughness, maraging steels provide resistance to embrittlement (hydrogen, neutron irradiation, and temper) and ease of fabricability. These advantages have led to consideration and use for a wide variety of applications. Important factors that have evolved since the original development are also cited. Control of certain processing factors, including purity level and ingot working practice, have improved the level of properties obtained from maraging steels. Better heat treatments have also provided improvements in both cast and wrought maraging steel. Current developments are outlined and predictions for future performance levels are made, based upon better current laboratory scale results. Significant progress in upgrading the strength-toughness combination of heavy sections, welds, and thin sheet are predicted. Prospects for more stainless grades are also promising. Growth into additional application areas is anticipated.     

Transkristalline Spannungsrißkorrosion an austenitischen Manganstählen in chlorionenhaltigen Angriffsmedien

Transcrystalline stress corrosion cracking of austenitic manganese steels in corrodent containing chloride ions Experimental investigation with steels (0,5% C, 20% Mn, 0.2—2.7% Cr) in the shape of U-bent specimens in seawater (RT and boiling) under loads between 30 and 45 kp/mm². The electrochemical behaviour of chromium-free types (i.e. below 0.5% Cr) at RT is identical to that of ferritic shipbuilding steels (equal break-through potential, then uniform attack). Addition of N, Ni, Co and Cu have little influence on the potential behavior. In boiling sea-water, however, transcrystalline corrosion takes place, accelerated by high tensional loads and anodic polarization, and slowed down by cathodic polarization. Additions, in particular of Co and Ni, reduce specimen life, perhaps by reducing yield strength. At higher Cr contents (1.7–2.8%) susceptibility to intercrystalline corrosion begins to supersede susceptibility to transcrystalline cracking. On the basis of ideas concerning the active-passive behavior under the influence of gliding phenomena the author shows analogies to the stress corrosion behaviour of austenitic stainless steels.     

18Ni马氏体时效钢强化方法概述

18Ni马氏体时效钢是以无碳（或超低碳）铁镍马氏体为基体的,主要是经时效产生时效强化的高强度钢。本文简要概述了18Ni马氏体时效钢的发展过程,介绍了固溶强化、相变强化、时效强化、细晶强化、形变强化方法和发展趋势。     

Die Rolle der Alkalisulfate bei der Hochtemperaturkorrosion. Teil II: Bildung von Sulfiden

The role of alkali sulfates during the high temperature corrosion Expermental work with various nickel base materials (NiCr20 Ti Al = Nimonic 80 A, NiCr20Co18TiAl = Nimonic 90) and high alloy steels (X40CoCrNi 20 20 20 = S 590, X8 CrNiNb 16 13 = ATS, X12 CrNiWTi 16 13 = 17/13W) and pure nickel. The action of Ni₃S₂ results in a heavy intergranular attack, accompanied by metal growth already at 650 °C. i.e. below the melting point of nickel sulfide. Under the action the sulfate melts (Na and K sulfates) the formation of nickel sulfide depends from the oxygen content of the atmosphere: with sufficient oxygen present there is practically no sulfide. Addition of NaCl to the melt increases the aggressivity of the latter in particular at higher temperature (800 °C). and may result in the complete destruction of the metal. Scale layers which may be present have no protective effect when they are porous and thus are not able to resist the penetration of the sulfate to the metal surface. The resistance of the alloy is primarily determined by the nickel content and decreases as the nickel content is increased; with comparable Ni contents the Co contents are an important criterion because Co containing scales appear to be easily penetrated by sulfates.     

Änderungen in der Verteilung des Schwefelgehaltes in hochlegierten hitzebeständigen Stählen nach der Oxidation

Changes in the distribution of sulphur in high-alloy heat-resistant steels after oxidation The oxidation of steels and alloys has so far revealed that at elevated temperatures and in the immediate proximity to the oxidic layer, a concentration gradient forms in respect of those elements that have a greater affinity to oxygen. In the case of current heatresistant steels we are primarily confronted with a reduction in chromium contents and a corresponding increase in the contents of the elements with a lower affinity to oxygen such ac nickel and iron. Recent research has however demonstrated that a redistribution of sulphur sets in as well. With samples where more substantial decrement of wall-thickness was established, a decarbonised layer formed on the surface, with an internal and intercrystalline oxidation. On its boundary, with the practically unaffected portion of samples, a considerable amount of manganese sulphides was found. As oxidation proceeded, manganese sulphides accumulated in the layer below the surface. It is highly probable that what happens here is a gradual forcing out of sulphur by oxygen, which predominantly diffuses further into the steel. This process has been established with both chromium steels of the Cr21 and Cr27 types, and austenitic chromiumnickel steels of the Cr18Ni10, Cr24Ni20 and Crl8Ni38. The increased contents of sulphur were confirmed not only by surface and point analyses by means of a microprobe, but also by the classical chemical analysis. For example, with the steel of the Cr18Ni38 type, after oxidation at 1100°C/4000 hrs., with a decrement in wall-thickness of 0,75 mm, an increase in sulphur was established in the surface layer, from 0,017 % S to 0,072 % S.     

Effect of Ni and Mn on the Mechanical Properties of 22Cr Microduplex Stainless Steel

The tensile properties of 22Cr–2Ni–4Mn–0.2N micro-duplex stainless steels with different Ni and Mn contents were investigated. Duplex stainless steels were vacuum induction melted and hot rolled, then annealed at 1,000–1,100 °C, at which temperature both the austenite and ferrite phases were stable. The volume fraction of the ferrite phase was markedly affected by the alloying elements of Mn and Ni; 1 wt% of Mn was equivalent to 0.4 wt% of Ni. All of the steels tested at room temperature showed the common strain-hardening behavior, while the steels tested at lower temperatures(-30 or-50 °C)showed a distinct inflection point in their stress–strain curves, which resulted from the transformation of the austenite to straininduced martensite. The onset strain(e0) of the inflection point in the stress–strain curve depended on the Md30 value of the steel. Testing at lower temperatures resulted in smaller e0 and consequently higher strengths and fracture strains(ef). The tensile behavior was examined from the perspective of austenite stability of the micro-duplex stainless steels with the different Ni and Mn contents.     

The influence of tempering temperature,minor alloying elements,and cathodic polarization on the low frequency fatigue resistance of 18% Ni maraging steel

The low frequency fatigue resistance of 18% Ni maraging steel (G110) under environmental conditions conducive to hydrogen embrittlement was altered by changes in the S, P, Mn and Cu minor element contents. The detrimental effect of increasing the S, P or Mn contents independently could be reversed when they were in combination, owing to beneficial interactions between Mn and S and Cu and P.Tempering the G110 maraging steel at higher temperatures (500–650°C) reduced the hardness and increased the amount of retained austenite, with a consequent improvement in the hydrogen embrittlement resistance.     

Corrosion resistance and mechanical properties of nickel-bearing ferritic stainless steels

Additions of nickel to ferritic steels containing 25–28% Cr and 2–4% Mo increased the impact toguhness especially when more than 2% Ni was present. The effect of nickel content increased up to 4% Ni, the largest addition studied. Steels stabilized with niobium had lower transition temperatures then did corresponding steels stablizied with titanium. Steels containing 4% Ni required annealing at 1050 C to avoid intermetalic compounds. It was also noted that nickel reduced the upper shelf energy in the Charpy impact test and eliminated a sharp transition from ductile to brittle behaviour. No definite effect of nickel on pitting potential was pound but steels in the series 25Cr-3.5 Mo-Ni-Ti consistenly had more noble pitting potentials and greater resistance to crevice corrosion than the 28 Cr-2Mo-Ni-Ti steels. Nickel contents of 1 or 2% tended to improve crevice corrosion resistance while larger nickel contents were somewhat ditrimental. Nickel strongly reduced critical current densities for passivity both in l N H₂SO₄ and in l N HCL and yielded corresponding increases in resistance to corrosion by these acids. Although 1% Ni or more caused the annealed steels to be susceptible to stress corrosion cracking in MgCl₂ boiling at 140 C, while the as-Welded steels containing 4% Ni did not crack in boiling 25% Nacl at pH 1.     

无钴马氏体时效钢电子束焊接接头组织与性能分析

对18Ni无Co马氏体时效钢进行了真空电子束焊接,用金相显微镜观察了焊接接头的组织形貌,并测定了焊缝区、热影响区、基体的显微硬度.结果表明,18Ni无Co马氏体时效钢组织为板条马氏体组织,材料焊接性能良好,焊缝区凝固组织为胞状树枝晶,熔合线附近热影响区晶粒发生了再结晶,晶粒长大明显.硬度分布有明显的规律性,焊缝区硬度最低,细晶区硬度最高,熔合线附近的热影响区,离熔合线越远,硬度值越高.在距熔合线2.5 mm处有一个马氏体与奥氏体两相混合的狭窄区域,硬度较其两侧有明显降低.     

18Ni马氏体时效钢原始奥氏体晶界显示方法

利用光学显微镜(蔡司)，在室温下，通过配制不同成分、浓度浸蚀剂和化学浸蚀方法，对经不同温度处理后的18Ni马氏体时效钢的原始奥氏体晶界显示方法进行了系统研究。结果表明：采用腐蚀性强、浓度高的浸蚀剂只能获得18Ni马氏体时效钢的金相显微组织，而采用体积比为4%的硝酸酒精溶液，对800~925 ℃固溶空冷后的18Ni马氏体时效钢试样进行化学浸蚀，能清晰地显示出18Ni马氏体时效钢试样原始奥氏体晶界，并可以进行准确的晶粒度级别数评定。     

Effect of Hardening and Tempering on Cyclic Crack Resistance of Maraging Steels

Regular features of variation of the characteristics of strength and cyclic crack resistance are determined as a function of stage decomposition of solid solution in tempering of hardened maraging steels. Results obtained for sparingly alloyed maraging steels 03Kh11N10M2T and 03Kh11N10M2T1 are used for considering the probable mechanisms of the effect dispersion and nature of intermetallic segregations on the growth of fatigue crack at different levels of the amplitude of the stress intensity factor ΔK.     

马氏体时效钢的热处理工艺及应用

马氏体时效钢是一种超高强度钢,是通过时效过程中从过饱和固溶体(马氏体)中析出Co、Mo、Ti等合金元素的碳化物实现强韧化。马氏体时效钢在具有高强度的同时还具有良好的塑性、断裂韧度、焊接性、冷热加工性和耐应力腐蚀性能。马氏体时效钢的热处理工艺比较简单,主要为退火、时效及形变热处理。马氏体时效钢已在航空航天、海洋工程、原子能工业及结构件、工模具等领域得到了广泛应用。     

高合金马氏体耐热钢的热变形表观激活能

采用Gleeble-3500试验机对0Cr11Ni2MoVNb、1Cr20Co6Ni2Mo1WV和403Nb高合金马氏体耐热钢进行热压缩试验,结果表明,3种钢的热变形表观激活能分别为461.6、508.1l和571.1 kJ/mol。通过同其他研究对比,发现影响钢的表观激活能的因素主要是碳含量和合金元素含量。在普碳钢以及微合金钢中,合金元素含量少,形成的碳化物少,对钢的表观激活能影响不大;对于高合金钢,一方面是由于碳化物,尤其是细小的NbC粒子对位错的钉扎作用,另一方面是由于合金元素的固溶强化作用,使得高合金钢的热变形表观激活能随钢中碳含量和合金元素含量的增加而增加。     

18Ni马氏体时效钢原始奥氏体晶界显示方法

利用光学显微镜(蔡司)，在室温下，通过配制不同成分、浓度浸蚀剂和化学浸蚀方法，对经不同温度处理后的18Ni马氏体时效钢的原始奥氏体晶界显示方法进行了系统研究。结果表明：采用腐蚀性强、浓度高的浸蚀剂只能获得18Ni马氏体时效钢的金相显微组织，而采用体积比为4%的硝酸酒精溶液，对800~925 ℃固溶空冷后的18Ni马氏体时效钢试样进行化学浸蚀，能清晰地显示出18Ni马氏体时效钢试样原始奥氏体晶界，并可以进行准确的晶粒度级别数评定。     

Effect of Cr Contents and Heat Treating on Reverted Austenite in Maraging Steel Weldments

By conducting flux cored arc welding (FCAW) on maraging steels with Cr contents of 1.4 and 5.2 wt%, this study observed the effects of Cr content and heat treating on reverted austenite formation in welded maraging steel. Aging treatment was carried out at the temperatures of 450, 480 and 530 °C for 3 h in each condition. As the aging temperature increased, reverted austenite was formed along the interdendritic and intercellular grain boundaries, and the proportion of reverted austenite increased with increasing Cr addition. The aging process led to the segregation of Ti and Mo along the interdendritic and intercellular grain boundaries. Some of the welded specimens were subjected to solution heat treatment at 820 and 1250 °C for 1 h after welding, resulting in a decrease in reverted austenite fraction.     

Solubility of Nitrogen in Superaustenitic Stainless Steels During Air Induction Melting

The amount of nitrogen contained in super austenitic stainless steels (SASS) influences their properties significantly. The effect of maximum amount of nitrogen in the highly alloyed Cr and Ni SASS containing further additions of Mo and Mn is studied. The calculated nitrogen contents of the experimental alloys are compared with the actual nitrogen contents obtained in the alloys produced using induction melting furnace. The actual nitrogen content of the alloys is always lower than the calculated value, and this discrepancy is due to the presence of positive interaction parameters of Ni, Cu, and Si in the alloy. However, the yield of nitrogen in the liquid SASS is improved significantly with additions of Mn and Mo contents. The construction of multicomponent phase diagrams for SASS is demonstrated using Thermo-Calc software. SASS containing more nitrogen exhibited a very high strength without loss of toughness.