Ultrahigh Strength Low Alloy Steel Research Articles

Effect of titanium modification on the fracture behavior of ultrahigh-strength low-alloy steel

The causes of instability of fracture toughness of ultrahigh-strength low-alloy medium-carbon steel 55KhN2MA-Sh are determined with the help of mechanical tests and scanning electron microscopy of fracture surfaces. It is shown that the fracture toughness changes due to a change in the concentration of titanium within the grade composition. When the concentration approaches the upper standardized limit, the ductility and the impact toughness of the specimens decrease due to excessive growth in the number of segregations of titanium carbosulfides.

Development of low alloy ultrahigh strength steel

Thermomechanical treatment (TMT) of plates is used to obtain the optimum combination of strength and toughness. Although this technique is applied to high strength low alloy (HSLA) steels, its application to ultrahigh strength steel (UHSS) is still under study. In this investigation, four C, Mn, Cr and Mo alloys, some with Nb or Ti additions, were prepared by electroslag refining (ESR), hot rolling with TMT and variations in cooling rate. Mechanical properties were evaluated and microstructural features were characterised. The oil cooled specimens of titanium alloy produced the optimum properties with higher strength values: ultimate tensile strength (UTS)=2177 MPa, yield strength (YS)=1795 MPa, elongation (El)=8% and impact toughness=713 kJ m−2, with microstructures predominantly consisting of lath martensite.

Study of welding characteristics of 0.3C–CrMoV(ESR) ultrahigh strength steel

A new ultrahigh strength low alloy steel 0.3C–CrMoV(ESR), having an ultimate tensile strength and 0.2% proof strength of above 1,700 and 1,500 MPa, respectively, in quenched and tempered condition, was developed primarily as a cost effective material for space launch vehicle applications. Welding is a major step in the fabrication of most of the pressure vessels, structures and equipments. Steels with carbon equivalent in excess of 0.40 wt% show a tendency to form martensite on welding, and therefore are considered difficult to weld. 0.3C–CrMoV(ESR) steel has a carbon equivalent value of nearly 1.0 that classifies it as a ‘very difficult to weld’ steel. In addition it has a niobium content of about 0.10% and a vanadium content of 0.25%. It is known that niobium content of more than 0.02 wt% has a deleterious effect on the toughness properties of low carbon welds. It has also been reported that the effect of niobium on weld metal toughness is more deleterious in the presence of vanadium. Hence, in the present study, the properties of the weldment of this new steel under different heat treatment conditions (HT-1 and HT-2) have been studied. In HT-1 condition, the plates were welded in hardened and tempered condition and no further heat treatment was given after welding, while in HT-2 condition, the annealed plates were subjected to welding followed by hardening and tempering heat treatments. For HT-1 condition, only tensile properties were evaluated. The welded plates under HT-2 condition were evaluated for tensile properties, fracture toughness, residual strength and microstructure features.

Corrosion characteristics of newly developed structural DMR-1700 steel and comparison with different steels for industrial applications

Corrosion characteristics of different steels in industrial environment were studied systematically to understand the protective nature of oxide scales that form on their surfaces with a view to explore the possibility of using a newly developed ultrahigh strength steel, DMR-1700, for fabrication of components to use in industrial applications. Further, the studies related to pitting and crevice corrosion resistance of a variety of steels under industrial environmental conditions have been carried out for comparison purposes. The surface morphologies of corroded steels were observed by scanning electron microscope (SEM) in order to understand the nature of corrosion. The corrosion mechanism of steel components that fail under industrial environmental conditions has been discussed. Based on the results obtained with different techniques, the newly developed DMR-1700 ultrahigh strength low alloy steel in association with successfully developed high-performance protective coating has been recommended for manufacture of components to use in industrial systems. This steel helps in improving the efficiency of the systems significantly by eliminating failures during service.

Effect of austenitizing temperature and cooling rate on the structure and properties of a ultrahigh strength low alloy steel

A 0.3C-CrMoV(ESR) steel is being developed primarily for making pressure vessels used for aerospace applications. Since it is important to understand the range of microstructures and mechanical properties that will be obtained in the heat affected zone of welds, the steel has been subjected to different austenitizing treatments (temperatures ranging from 925°C to 1250°C) followed by cooling at various rates to room temperature. It has been shown that the austenite grain size increased from about 10 to 250 μm as the austenitizing temperature is increased from 925°C to 1250°C (1 hr) and that the hardness, YS, UTS,% elongation and% reduction in area as well as CVN energy for 450°C tempered condition decrease as the austenitizing temperature is increased for all cooling rates (furnace cooling, air cooling, oil quenching, quenching and tempering at 450°C). This is attributed mainly to the increase in austenitic grain size. The ranges of microstructures that can be obtained in the heat-affected zone are massive ferrite, fine pearlite, upper as well as lower bainite and martensite. The Charpy impact energy for the oil-quenched steel tempered at 200°C, however, did not vary significantly with austenitizing temperature.

Study of fracture properties of 0·3C–CrMoV(ESR) ultrahigh strength steel

The fracture properties of a new ultrahigh strength low alloy steel 0·3C–CrMoV(ESR), developed primarily as a cost effective material for space launch vehicle applications, have been evaluated through two different methods. The steel was processed through electroslag remelting (ESR) with inoculation using niobium to produce 4 tonne ingots, which were forged and rolled into plates. The fracture toughness of this steel has been evaluated by testing compact tension specimens and surface cracked tension specimens following the procedures in ASTM E–399 and ASTM E–740 respectively. The steel, after quenching and tempering, has a 0·2% proof strength of 1470 MPa, ultimate tensile strength of 1725 MPa and fracture toughness of 96 MPa √m. It has high residual strength also in the presence of surface cracks of 5 × 2 mm size. The microstructure consists of fine grains in the annealed condition and lath martensite in the hardened condition. After tempering, fine carbide precipitates distributed along the lath boundaries and within the laths were observed.

Improvement of Low Temperature Mechanical Properties of Ultrahish-strength Low-alloy Steels

Ultrahigh-strength low-alloy steels with a medium carbon (0.25 to 0.50 mass%) content and various amounts of chromium, molybdenum, nickel, silicon, and vanadium have been needed for high-performance aerospace parts. The steels can be employed successfully at yield strengths of 1400 MPa or higher, but their commercial use is limited by their poor ductility and toughness at low ambient temperatures. Brittle fracture has to be given serious consideration at these levels. This review surveys improvement of the low temperature mechanical properties of ultrahigh-strength low-alloy steels. Particular emphasis is placed on improving the mechanical properties through mixed-structure control, thermomechanical treatments, and modification of non-metallic inclusions. The major microstructures controlling the mechanical properties are discussed for each of these techniques.

低合金超強力鋼の低温機械的性質の改善

Ultrahigh-strength low-alloy steels with a medium carbon (0.25 to 0.50 mass%) content and various amounts of chromium, molybdenum, nickel, silicon, and vanadium have been needed for high-performance aerospace parts. The steels can be employed successfully at yield strengths of 1400 MPa or higher, but their commercial use is limited by their poor ductility and toughness at low ambient temperatures. Brittle fracture has to be given serious consideration at these levels. This review surveys improvement of the low temperature mechanical properties of ultrahigh-strength low-alloy steels. Particular emphasis is placed on improving the mechanical properties through mixed-structure control, thermomechanical treatments, and modification of non-metallic inclusions. The major microstructures controlling the mechanical properties are discussed for each of these techniques.

低合金超強力鋼のオーステンパー組織と機械的性質に及ぼすＳｉ添加の影響

Silicon-modified 4340 (Si-4340) and 4340 steels have been studied to determine the effects of silicon addition on austempered structure and mechanical properties of ultrahigh strength low alloy steels. As a result of austempering Si-4340 steel in the bainitic temperature region (593-673K), the retention of a large amount of austenite (12-25vol.%) was observed in the carbon-free upper-bainitic ferrite region. Compared to the conventionally quenched and tempered steel (CQT), the austemper of Si-4340 steel increased fracture toughness (KIC) at 623K and below, owing to the increased Charpy impact energy and percent elongation, while the strength decreased. Compared to the CQT and 4340 steel austempered in the same temperature region, the austemper of the steel had detrimental effects on the KIC and other relevant mechanical properties at 643K and above. The results are described and discussed in terms of metallographic observations, X-ray measurements and fractography.

Morphology control of ductile second phase and improved mechanical properties in high-strength low-alloy steels with mixed structure

Mixed structures of martensite and non-martensitic decomposition product (ductile second phase) can often be produced in commercial heat treatments of low-alloy steels. Such mixed structures can sometimes produce a detrimental effect on the mechanical properties of steels, but they can significantly improve strength, ductility and notch toughness if the ductile second phase appears in a suitable morphology (size, shape and distribution) in association with tempered martensite. Therefore, microstructures have recently been produced in a deliberate attempt to improve the mechanical properties of ultrahigh-strength low-alloy steels. In this review, an attempt has been made to present the effect of the morphology (shape, size and distribution) of the ductile second phase on improved mechanical properties of ultrahighstrength low-alloy steels having mixed structures, This review first discusses the effect of the morphology of the second-phase bainite on mechanical properties of high-strength low-alloy steels with mixed structures of martensite and bainite, in which great improvement in the mechanical properties of the steel has been associated with the morphology of the bainite. Then, knowledge of the recent development of a steel having the mixed structures for low-temperature ultrahigh-strength applications is also reported.

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Recently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of ≥1400 MN m−2 but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.MST/1413

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Effect of hot-rolling reduction on shape of sulfide inclusions and fracture toughness of AISI 4340 ultrahigh strength steel

Commercial AISI 4340 ultrahigh strength steels with hot-rolling reductions of 80 to 98 pct have been studied to determine the effect of the shape of sulfide inclusions on plane-strain fracture toughness(KIC) of the ultrahigh strength low alloy steels. The significant conclusions are as follows: decreasing the hot-rolling reduction from 98 to 80 pct for the steels modified the shape of sulfide inclusions from the stringer (average aspect ratio = 17.5) to the ellipse (average aspect ratio = 3.8). This improved theKIC in the longitudinal testing orientation by about 20 MPa · m1/2 at similar strength levels. This could be due to the fact that the ellipsed sulfide-inclusions separate from the matrix during plastic deformation, producing large voids. During testing these act to blunt and arrest cracks propagating across the specimen which would normally cause failure. The decrease in the hot-rolling reduction also developed theKIC in the transverse testing orientation by about 17 MPa · m1/2 at increased ductility and Charpy impact energy levels. This can be attributed to the fact that lamellate fracture, which occurs in a brittle manner along the interfaces of the sulfide-inclusion/matrix at the crack tip, is considerably suppressed by modifying the shape of the inclusions from the stringer to the ellipse.

Modified heat treatment for lower temperature improvement of the mechanical properties of two ultrahigh strength low alloy steels

In the previous papers, a new heat treatment for improving the lower temperature mechanical propertise of the ultrahigh strength low alloy steels was suggested by the authors which produces a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite through isothermal transformation at 593 K for a short time followed by water quenching (after austenitization at 1133 K). In this paper, two commercial Japanese ultrahigh strength steels, 0.40 pct C-Ni-Cr-Mo (AISI 4340 type) and 0.40 pct C-Cr-Mo (AISI 4140 type), have been studied to determine the effect of the modified heat treatment, coupled above new heat treatment withγ ⇆ α′ repctitive heat treatment, on the mechanical properties from ambient temperature (287 K) to 123 K. The results obtained for various test temperatures have been compared with those for the new heat treatment reported previously and the conventional 1133 K direct water quenching treatment. The incorporation of intermediate four cyclicγ ⇆ α′ repctitive heat treatment steps (after the initial austenitization at 1133 K and oil quenching) into the new heat treatment reported previously, as compared with the conventional 1133 K direct water quenching treatment, significantly improved 0.2 pct proof stress as well as notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel at similar fracture ductility levels from 287 to 123 K. Also, this heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved both 0.2 pct proof stress and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel with increased fracture ductility at 203 K and above. The microstructure consists of mixed areas of ultrafine grained martensite, within which is the refined blocky, highly dislocated structure, and the second phase lower bainite (about 15 vol pct), which appears in acicular form and partitions prior austenite grains. This newly developed heat treatment makes it possible to modify the new heat treatment reported previously so as to raise 0.2 pct proof stress to a higher level and keep notch toughness at the same level. The improvement in the mechanical properties is discussed in terms of metallographic observations and the modified law of mixtures and so forth.

Low temperature improvement of the mechanical properties of 4340 type ultrahigh strength steel with heat treating techniques using interrupted quenching method

In many practical applications the use of ultrahigh strength low alloy steels, such as AISI 4140 and 4340, is limited by their poor ductility and notch toughness in low temperature environments. In recent years, considerable effort has been directed toward improving ambient temperature mechanical properties by various heat treating techniques or compositional modification. ~-~2 Unfortunately, these improvements in mechanical properties have not been reflected in the low temperature mechanical properties of the ultrahigh strength low alloy steels. The basic problem is that the martensitic microstructure tempered to retain a high strength level is inherently susceptible to embrittlement which occurs in low temperature environments. We have given one of the potential solutions to this problem by developing 4140 and 4340 type ultrahigh strength steels having a mixed structure of martensite and lower bainite. 13.14,15 The effectiveness of the mixed structure in improving the mechanical properties is attributed to the fact that lower bainite, which appears in acicular form and partitions prior austenite grains, effectively causes a refinement of the martensitic substructure (i .e. , lath width and packet size) which lead to increased strength and that the lower bainite provides a significant increased resistance to brittle fracture at low temperature environments. As noted, the role of the second phase lower bainite in improving the lower temperature mechanical properties has been found to involve a refinement of the martensitic substructure and increasing resistance to low temperature brittle fracture. Thus, it follows that other ductile second phases should have a similar beneficial effect, provided they appear in acicular form and refine the substructure of the parent martensite, resulting in a higher resistance to cracking in low temperature environments. The obvious alternative second phase to consider is highly tempered martensite. This has the great advantage that the required microstructure can be readily achieved by heat treating techniques using interrupted quenching method. 16'17 Also, there is an added advantage that if the resulting microstructure improves the mechanical properties, the heat treating techniques can be applied to larger sections than the new heat treatment reported previously, ~3'Ja''s because the formation of the microstructure predominantly depends upon temperature.

Microstructural analysis of a multistage heat-treated ultrahigh strength low alloy steel

The microstructure that results from a modified heat treatment process for 300M steel (silicon-modified AISI 4340 steel) was extensively studied. Cyclic heat treatment resulted in a higher yield strength than that obtained by conventional heat treatment, but the plane strain fracture toughness decreased. The introduction of an intermediate tempering step prior to the re-austenitization step solved this problem and all investigated properties were improved. This modified heat treatment process resulted in the refinement of prior austenite grain size and lath width, a reduction in the amount of twinned plate martensite, an increase in the amount of interlath films of retained austenite and a fine dispersion of the carbides M 23C 6 and M 7C 3 in addition to cementite and ϵ carbide. The fracture mode in the initiation region also changed from predominantly quasi-cleavage to a mixture of quasi-cleavage and microvoid coalescence, with a smaller void size. The interplay of these microstructural changes resulted in improved mechanical properties.

Observations of a grain boundary phase in laser-heat-treated ultrahigh strength low alloy steels

Thin foil transmission electron microscopy studies of laser-transformation-hardened martensitic microstructures of ultrahigh strength low alloy steels, such as AISI 4340 and 300-M, revealed a grain boundary phase similar to that obtained in conventionally heat-treated and quenched samples but which had a different substructure. This phase essentially contained dislocations. Selected area diffraction patterns coupled with dark field microscopy studies confirmed the absence of other structural features including auto-tempered carbides and twins within this phase. The irregular morphology, the lack of substructural features other than dislocations of this phase and its presence only at the grain boundaries suggest that this phase is a massively transformed ferrite. In contrast, this grain boundary phase had previously been identified as either bainite or “blocky” martensite in conventionally heat-treated and quenched samples.

Achieving optimum properties in ultrahigh-strength low-alloy steel

The effects of cyclic austenitization treatments, coupled with intermediate tempering, on the mechanical properties of 300M steel have been studied. The results obtained for these modified heat treatments have been compared with those for the conventional 1143 and 1473 K austenitization treatments. Although cyclic treatment refined the prior austenitic grain size and increased the yield strength, the plane-strain fracture toughness decreased. The introduction of an intermediate tempering treatment between the two-cycle austenitization treatments improved plane-strain fracture toughness as well as the yield strength, while the impact toughness and ductility either remained the same or increased. The improvement in the mechanical properties is attributed to prior austenitic grain-size refinement and to precipitation of carbides during the intermediate tempering stage and the subsequent partial dissolution of these carbides during re-austenitization

CRUCIBLE D6

Abstract Crucible D6 is a low alloy ultra-high strength steel developed for aircraft-missile applications and primarily designed for use in the 260,000-290,000 psi tensile strength range. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties as well as fracture toughness, creep, and fatigue. It also includes information on low temperature performance as well as forming, heat treating, machining, and joining. Filing Code: SA-129. Producer or source: Crucible Steel Company of America.