Special features of the formation of microstructure of heat-treated low-alloy steel

The strength and the operational properties of low-alloy thick-sheet steels is increased by heat treatment in the line of the mill with the use of the rolling or furnace heat. The present work concerns the special features of structure formation and the mechanical properties of rolled stock from sheet structural steel 09G2S with a strength level C345 (σ_y=325-345 MPa) in a hot-rolled and a normalized state after accelerated cooling from the normalizing furnace and after rolling behind the finishing stand. As a result of the rapid cooling in the line of the mill the sheet acquires properties corresponding to steels with a C390–C440 strength level (σ_y=390-440 MPa). Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 9–11, August, 1998.     

In-flow hardening of low-carbon steel

Conclusions 1. Shapes from low-carbon steels of the St3 type can be hardened to σ_y≥500 MPa in a line of small- and medium-section mills by the method of interrupted quenching by turbulent water streams. 2. High service and technological properties are provided in the rolled stock with σ_y≥500 MPa under the condition that the surface hardness is limited to ≤320HV. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 36–38, September, 2000.     

Einflußgrößen der H-induzierten Spannungsrißkorrosion bei niedriglegierten Stählen

Parameters influencing the hydrogen-induced stress corrosion cracking of low alloy steels Wet H₂S corrosion of steel leads to surface blistering, internal cracking at sharp edged inclusions (HIC), and in stressed specimens to transgranular micro cracks transversal to the main stress. The latter grows to SCC. HIC resistant material displays high purity of non-metallic inclusions. The critical stress level for SCC of lien pipe steels with yield strengths up to 600 N/mm² is proportional to the yield strength. In H²S containing media different grades of low alloy and rolled steels show no correlation between life time and yield strength. In general, the susceptibility for H-induced SCC increases with decreasing pH and with cathodic polarisation, whereas the temperature dependence displays a minium of life time at 20°C.     

Wasserstoff-induzierte Spannungsrißkorrosion von Stählen durch dynamisch-plastische Beanspruchung in Promotor-freien Elektrolytlösungen

Hydrogen induced stress corrosion cracking of steels subjected to dynamic loading involving plastic deformation in promotor free electrolytic solutions Plain carbon steels and low alloy steels suffer internal cracking and a relatively high embrittlement when they are subjected to dynamic loading involving plastic deformation in any type of electrolytic solution where there is a simultaneous cathodic hydrogen evolution. These conditions can be encountered in service in the case of cathodic polarisation and free corrosion in acids if the mechanical stresses lead to plastic deformation, e.g. at notches. There is an upper limit to the potential range in which internal cracking occurs. This limit is independent of the yield strength (300 to 500 N mm^?2) of the materials tested and lies at U_H = ?0.5 V in oxygen free salt waters. It is more negative in oxygen bearing electrolytic solutions or in alkalaine media. Materials containing hard transformation products such as martensite and bainite are more susceptible to cracking in the regions of these hard transformation products. There is only a small decrease in susceptibility with increasing temperature. Ultra high strength, quenched and tempered steels with yield strengths > 1000 N mm^?2 undergo hydrogen induced stress corrosion cracking even when they are subjected to static loading in the elastic region. The critical potential is very negative and shifts to less negative values with increasing strength of the material. Stainless steels with stable austenitic microstructure are resistant to this type of corrosion. However, if the mechanical deformation can lead to the formation of martensite surface cracking and brittle fractures occur. The hydrogen induced damage decreases with increasing temperature.     

Optimization of aging regimes of alloys in the Al−Mg system for stabilizing the structure and mechanical properties

The previous study of the authors has resulted in the development of an Al−10% Mg composition alloyed additionally by zinc, manganese, antimony, and zirconium. The alloy was to be cast into sand molds and had σ_r=380-400 N/mm², σ_0.2=240-260 N/mm², and δ=10–12%. The main difficulty in the development and use of the given alloy consisted in attaining stable properties in the operating process. The present paper is devoted to this problem. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 32–34, August, 1997.     

High-strength structural steels with a structure of low-carbon bainite subjected to thermomechanical treatment

Conclusions The development of TMT for low-pearlite and pearlite-free steels has led to the development of steels containing 0.06% C, 1.8% Mn, 0.3% Mo, and 0.05–0.09% Nb. Depending on the type of TMT, the most important parameters of which are the final rolling temperature and the total deformation at temperatures below 900°, the transformation occurs partially or completely in the intermediate region. The high dislocation density of the bainite has a substantial effect on the increase of the yield strength.Large rolling reductions at temperatures below 870° favor the formation of ferrite nuclei. The polygonal ferrite formed in this case has a characteristic grain size of grade 13–14.Rolling under laboratory conditions showed that by selecting the conditions of TMT one can control the mechanical properties of the steel. A guaranteed yield strength of 70 kgf/mm² and transition temperature T₅₀=–25° can be obtained on plates 13 mm thick, while after rolling under other conditions the ductile-brittle transition temperature is –125° and the yield strength 45 kgf/mm². Numerous production tests indicate that a yield strength of at least 50 kgf/mm² in the hot rolled condition can be obtained for plates 20 mm thick. In rolling mills permitting substantial reduction at low temperatures these values can be improved. For example, a yield strength of 56 kgf/mm² and transition temperature of –60° can be obtained for plates 30 mm thick. The steel has substantial reserves of strength due to precipitation hardening, which amount to 8–12 kgf/mm² after tempering at 600–625°. The weldability of the steel is good due to its low carbon content.Stahl und Eisen,93, No. 22, 1041 (1973).Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 11–18, December, 1975.     

High-temperature thermoplastic strengthening of steels St3sp and 09G2S

Conclusions We worked out a regime of high-temperature thermoplastic treatment of steels St3sp and 09G2S consisting in austenization at 1020°C, strain (=200%, =5.7%) at 940°C with subsequent cooling at a rate >100°C/sec.As a result of such treatment the tensile strength of steel St3sp is increased to 1230 N/mm², of steel 09G2S to 960 N/mm², and the conventional yield strength to 1100 and 840 N/mm², respectively.N. É. Bauman Moscow State Technical University. All-Union Research Institute of Metallurgical Machinery Construction. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 18–19, July, 1991.     

Optimizing heat treatment regimes for high-strength weldable, sparingly alloyed steels

It is important to devote maximum attention to the heat treatment of temperable high-strength weldable steels (σ_0.2≥700 MPa) in which the expensive alloying elements (Cr, Ni, Mo, etc.) are partially or fully replaced by microadditives (V, Ti, etc.). The present paper concerns the effect of the temperature-and-time heat treatment parameters on the structure, the grain size, and the mechanical properties of sparingly alloyed steel 15G2KhNMFTR. The heat treatment regime is optimized. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 2–5, August, 1998.     

Assessment of the electrochemical corrosion properties and environmentally induced cracking of an A106 Gr B steel pipe weld in a NaCl solution saturated with H2S gas

In the area of heavy construction, welding processes are vital in the production and maintenance of pipelines and power plants. The fusion welding process generates formidable welding residual stress and metallurgical change, which together increase the crack driving force and reduce the resistance against brittle fracturing and environmental fracturing. This is a serious problem with many alloys, and it also arises in A106 Gr B steel pipes. This type of piping, which is used in petrochemical and heavy chemical plants, either degrades due to the corrosive environment, e.g., those containing chlorides and sulfides, and/or become damaged during service due to various corrosion damage mechanisms. Thus, in this study, after numerical and experimental analyses of the welding residual stress of a multi-pass welded A106 Gr B steel pipe weld, the electrochemical corrosion properties and environmentally induced cracking of an A106 Gr B steel pipe weld were assessed in a 5.0 wt.% NaCl solution that was saturated with H2S gas at room temperature on the basis of NACE TM 0177-90. In terms of sulfide stress corrosion cracking (SSCC) and sulfide corrosion fatigue (SCF), the low SSCC limit of smooth specimens, σ_SSCCsmooth, and the SCF limit, Δσ_SCF, were 46 % and 32 % (160 MPa) of the ultimate tensile strength (502 MPa) of an A106 Gr B steel pipe weld, respectively. Further, (Δσ_res)_SCF was assessed under 75 MPa, which is 15 % of the tensile strength.     

Two-phase ferritic-martensitic steels

Conclusions The results of laboratory and production experiments showed that the main condition for providing the necessary combination of properties of two-phase ferritic-martensitic steels (high tensile strength, low yield strength, high plasticity and work hardenability) is obtaining the specified quantity of the hardening phase (martensite) in the structure (20–28% M to obtain _t 550 MPa or 10–18% M for _t 450 MPa). The specified ratio of the structural constituents under conditions of mass production of two-phase ferritic-martensitic steels may be guaranteed only with the use of steels containing carbon and alloy elements within the necessary limits and also with strict observance of the heat-treatment cycle. Without the use of metallographic measurements as the criteria for obtaining the optimum structural condition in addition to the required values of strength and plasticity it is necessary to use the _0.2/_t ratio, which must not be greater than 0.5–0.6 with the absence of yield points on the tensile curve (without special temper rolling).As the result of the combination of work done in the Central Scientific-Research Institute of Ferrous Metallurgy together with plants of the Ministries of Ferrous Metallurgy and of the Automobile Industry at present the production is being introduced and experimental production lots of heat-treated two-phase steels of the following types with guaranteed mechanical properties are being supplied:At the Novolipetsk Metallurgical Combine cold-rolled 0.7–2.0-mm sheet of 06KhG(S)Yu and 06G2SYu steels with _0.2 = 260–320 MPa, _t 550 MPa, ₄ 30%, and _0.2/_t 0.5–0.6.At the Cherepovets Metallurgical Combine hot-rolled 2–6-mm sheet of 09G2(S) steel with _0.2 = 260–320 MPa, _t 550 MPa, ₄ 25%, and _0.2/_t 0.5–0.6.At the Beloretsk Metallurgical Combine (billets melted and rolled at the Cherepovets Metallurgical Combine) heat-treated cold-drawn wire up to 10.5 mm in diameter of 06KhGR steel with _0.2 250 MPa, _t 530 MPa, and ₁₀ 30%.For successful development of the production and use of two-phase ferritic-martensitic steels the combined work of the designers of machine building plants on determination of the most effective types of parts, of engineers on correction of the method taking into consideration the specifics of the properties of two-phase ferritic-martensitic steels, and of users on determining the service properties of parts of these steels is necessary. The fulfillment of such a combination of work will make possible th timely formulation of the further work of the metallurgical industry and determination of the required volume and product range of rolled products of two-phase ferritic-martensitic steels.I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 25–29, November, 1984.     

Transformation hardening of steel sheet for automotive applications

Among high-strength steels, transformation hardening steels such as dual-phase (DP) steel and transformation-induced plasticity (TRIP) steel offer a superior relationship between tensile strength (TS) and elongation (El) on a commercial scale. As demand has grown for lighter-weight automobiles, so also has the demand for higher TS, lower yield ratio, and higher hole expansion ratio grown. Recently DP steel has been developed with precipitation hardening and grain refining by TiC. A new TRIP steel composed of 5Mn-2Si and control-rolled with niobium addition suggests the formation of retained austenite (γ _R ) as much as 30% and TS × El = 3,000 kgf/mm²·%.     

Der Einfluß der Umgebung auf die Rißzähigkeit und Rißausbreitung bei höherfesten Stählen

Influence of the environment on the cracking toughness and crack propagation in high-strength steels The crack toughness of four fine grained steels and one high strenght steel of different heat-treatments are investigated in 3% NaCl solution depending from the electrochemical polarisation. The steels are of commerical compositions, their strengths are in the range of 635 to 1460 N/mm². The stress intensity factors, K_ISCC for slow crack growth are determined by a method of fracture mechanics. For these investigations the DCB-specimen with an incipient crack is used. The K_ISCC-values generally decrease with increasing strength of the steels, but they show different susceptibility for crack growth under cathodic and anodic polarisation. Outdoor experiments conducted in seawater covering a period of one year confirm the results of laboratory.     

The effect of alloying with molybdenum on the corrosion—cracking resistance of a Γ20Φ2 type manganese steel against

Molybdenum, which is introduced in amount of 2 to 6% into manganese steel of the basic composition (0.15–0.25) Γ20Φ2, enhances its strength upon the maximum age-hardening at 650°C (to σ_y). However, its stress-corrosion cracking resistance in a 3.5% NaCl solution increases insufficiently (1500 h at a stress of 40 to 50% breaking stress in air σ_air). After the overaging (prolonged exposures at 750°C), steel (with any aforementioned molybdenum content) does not fail in the test medium in 4000 h at σ = 0.8σ_air.     

Mechanical properties of 32Mn-7Cr-0.6Mo-0.3N austenitic steel for cryogenic applications

The tensile, impact, and fracture toughness tests from ambient temperature to 77 K were carried out on 32Mn-7Cr-0.6Mo-0.3N austenitic steel. The fracture surfaces and the phase constitution were analyzed using scanning electron microscopy and x-ray diffraction. The results show that the relation between yield strength and temperature is σ_0.2 · 300 + 1392.4 exp (−0.0106T). The 77 K yield strength is 883 MPa · m^1/2 and the K _J0.05 value is about 236 MPa · m^1/2. The cryogenic intergranular fracture is fully suppressed. The 77 K fracture surfaces exhibit a tough character composed of many dimples and few small quasi-cleavage facets. The results of x-ray analysis show that the austenite phase of the steel is stable even under cryogenic deformation conditions.     

Effect of special features of distribution of nonmetallic inclusions in the metal of electric-welded oil conducting tubes on resistance to hydrogen-stress cracking

Results of an analysis of fracture surfaces obtained by the method of eccentric detachment of specimens of the metal of welded tubes from steel 09GSF in the initial state and after testing for hydrogen-stress cracking in a hydrogen sulfide medium are presented. Qualitative dependences of the susceptibility of the steel to hydrogen-stress cracking and of the kind of fracture in the initial stage and after testing for hydrogen-stress cracking on the purity of the metal with respect to nonmetallic inclusions in the zone of axial segregation are established. The dependences are used for developing a method for diagnosing rolled stock.     

Untersuchung zur phasengrenzseitigen Hemmung der Wasserstoffeffusion bei wasserstoffbeladenen Vergütungsstählen

Investigation into the effusion behaviour of hydrogen in quenched and tempered steels The effusion behaviour of hydrogen in quenched and tempered steel was investigated by means of the ultrasonic damping technique (USDT). The material 34CrMo4 was used in three tempering conditions (I: 850 N/mm², II: 1000 N/mm² and III: 1200 N/mm²). The influence of effusion behaviour of different media (imidazole, imidazolidine and EDTA) in dependence of the exposure time was determined. Additionally volumetric permeation experiments and the determination of the hydrogen embrittlement-index (V_H) were performed. The results revealed, that the USDT basically is suitable for the quantitative, non destructive measurement of hydrogen in quenched and tempered steel. The investigations showed a significant influence of different parameters on the reproducibility of the results. Especially the influence of media, the microstructure and the coupling of the testing button on the material surface are important for the reliability of the results by using the USDT under practical conditions.     

Phase transformations in heat treatment of Al−Li alloys and optimization of operational properties of semifinished products from these alloys

In recent years Al−Li alloys have become very popular in the aircraft industry. These alloys are characterized by a high specific strength and rigidity which increases substantially the load ratio and fuel efficiency of the parts. As a rule, Al−Li alloys are used in an underaged state, which provides a high, ductility over the height of the semifinished products. However, the threshold stress (σ_cr) in corrosion cracking in this state does not usually exceed 25–50 N/mm², and the yield strength is not always satisfactory. The results of a study of the mechanical properties and corrosion resistance of some Al−Li alloys are described and the ways for optimizing these properties are described on the basis of the phase transformations in the alloys. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 24–30, January, 1998.     

Einfluß der Festigkeit unlegierter und niedriglegierter Stähle auf H-induzierte Korrosionserscheinungen bei zügiger plastischer Belastung

Influence of mechanical strength on hydrogen-induced corrosion effects on unalloyed and low-alloysteels subjected to slow strain rate tests Three steels were subjected to various heat treatments or cold working to produce 7 variants of strength. Specimens from these materials were tested in 9 aqueous solutions containing various acidic components by the constant strain rate technique under cathodic polarisation or free corrosion conditions. Hydrogen induced cracks were only observed after passing the maximum load. Hydrogen induced cracking and the decrease in the reduction of area (hydrogen embrittlement) show a good correlation with the mechanical properties of the materials. For less ductile materials the number of cracks decreased with increased embrittlement. The magnitude of hydrogen embrittlement depends on the concentration of undissociated acid in the test solution and is independent of pH value. O₂ can reduce the embrittlement. The effect of test solution composition decreases as the cathodic polarisation potential becomes more negative, because hydrogen will then be generated from H₂O. There is no correlation between the type of corrosion effects and the strength of the material, except in the case of highly sensitive high strength variants with R_m in excess of 1000 N/mm². Quenched and tempered low alloy steels, even at high strength levels, have significantly higher resistance to hydrogen embrittlement than significantly higher resistance to hydrogen embrittlement than unalloyed steels. The same is also ture for workhardened variants. For unalloyed steels, metallurgical cleanness seems to have a favourable influence. No hydrogen induced corrosion effects were observed in specimens tested at 907°C under free corrosion conditions.     

Effect of warm extrusion on structure and properties of steel 20Kh

Conclusion Warm extrusion of steel 20Kh at 600–800° makes it possible to obtain a structure with high strength and ductile characteristics: _0.2=40–50 kg/mm², _b=50–60 kg/mm², =60–75%.Voroshilovgrad Machine Construction Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 56–57, March, 1976.