The role of Cu and Al addition on the microstructure and fracture characteristics in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels with superior toughness

The effects of Cu and Al addition on the microstructure and fracture in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels with superior toughness were studied and compared with the X70 pipeline base steel counterpart. The microstructure in base steel was dominated by a small fraction of acicular ferrite and predominantly bainite. However, acicular ferrite microstructure was obtained in Cu-bearing steel, which nucleated on complex oxide with outer layer of MnS and CuS because of Cu addition. The microstructure in Al-bearing steel consisted of bainite with ultrafine martensite–austenite constituent, which was refined by Al addition. CGHAZ in Cu-bearing and Al-bearing steels had superior impact toughness and ductile fracture, which were attributed to acicular ferrite and ultrafine martensite–austenite constituent, respectively.     

Development of mechanical properties of structural high-carbon low-alloy steels through modified heat treatment

The modified heat treatment, which produces a mixed structure of martensite and lower bainite through short-term isothermal transformation at just above the martensitic transformation temperature,M _s temperature, followed by oil quenching (after conventional austenitization), has been applied to three high-carbon low-alloy steels with different levels of nickel and chromium contents at similar molybdenum levels, in which carbon was allowed to replace relatively expensive additions of nickel and chromium, for their ultra-high strength application. The significant conclusions are as follows: an ultra-high strength steel of 1900 M Pa yieldstress grade with a high toughness level can be obtained when about 60 vol % lower bainite is associated with 473 K tempered martensite of 0.60% C-1.80% Ni-0.80% Cr-0.25% Mo steel. If approximately 25 vol % lower bainite appears in 673 K tempered martensite of the steel, a 1700 M Pa yield-stress grade steel with high toughness and moderate ductility levels can be attained. However, alloying nickel is essential to some extent for development of the mechanical properties with the modified heat treatment suggested in the present work.     

Role of recrystallization and pearlite dissolution in industrial processing of DP590 steels

Dual-phase (DP) steels derive their perfect blend of properties via the hard second phase, namely martensite or bainite in a softer ferrite matrix. The key to refine the mechanical properties of DP steels rests on optimizing and tailoring the distribution and size of the hard second phase present in the ferrite matrix. There can be several combinations of processing routes depending on the governing mechanisms, such as recrystallization, pearlite dissolution, phase transformation, etc., which can affect the morphology and distribution of martensite phase present in DP microstructures. All these mechanisms are invoked at various stages of annealing process cycle. In the present study, experimental simulation of various annealing parameters was carried out on a cold-rolled steel using a custom designed annealing simulator. The evolution of microstructure was studied by field emission scanning electron microscope. The evolving microstructures were correlated with governing mechanisms of recrystallization, pearlite dissolution, and phase transformation. Through these simulations, it was possible to tailor the microstructure and consequently improve the tensile properties of the DP steel.     

Improved fracture toughness of ultrahigh strength steel through control of non-metallic inclusions

In recent years, ultrahigh-strength steels, which can be employed successfully at yield strengths of 1400 MPa or higher, have been used increasingly for critical structural applications in aircraft and aerospace vehicles. Most recently, there has been increased demand, however, for ultrahigh-strength steel with superior plane-strain fracture toughness, K _IC, and for the steels suitable for large-sized structural applications; isotropy regarding the property has especially been required. One potential solution to this problem is to control nonmetallic inclusions of the steels. This review concentrates on recent topics concerning improved K _IC of ultrahigh-strength steels, i.e. low-alloy and highly alloyed secondary hardening steels, through control of non-metallic inclusions. The major factors controlling the property are discussed for each of the techniques.     

Microstructure,mechanical properties and high stress abrasive wear behavior of air-cooled MnCrB cast steels

Three medium carbon low alloyed MnCrB cast steels containing different Cr contents (0.3%, 0.6%, and 1.2%) were designed and the effect of Cr contents on the microstructure, mechanical properties and high stress abrasive wear behavior of the cast steels after 850 °C air-cooling and 220 °C tempering was studied. The results show that the hardenability of the MnCrB cast steels was excellent. The microstructure of the cast steels with low Cr contents (0.3% or 0.6%) consists of granular bainite and lower bainite/martensite multiphase. With increasing of Cr content, the formability of martensite was improved, the hardness and wear-resistance increased, but the impact toughness decreased in that more bainite was replaced by martensite. The air-cooled MnCrB cast steel containing 0.6% Cr, with granular bainite and lower bainite/martensite multiphase, exhibited excellent combination of strength, hardness, ductility, and impact toughness. In addition, its abrasive wear-resistance was 30% more than that of Hadfield cast steel in the high stress abrasive wear condition. This air-cooled MnCrB cast steel by simple alloying scheme and heat treatment has the advantages of high-performance, low cost, and environmentally friendly. It is a potential advanced wear-resistant cast steel for low- or even medium-impact abrasive conditions.     

Impact and tensile properties of ferrite–martensite dual‐phase steels

The effect of martensite morphology on the impact and tensile properties of dual phase steels with a 0.25 volume fraction of martensite (V_m) under different heat treatments was investigated. These treatments are direct quenching (DQ) and step quenching (SQ) that result in different microstructures and mechanical properties. To process dual phase steels, a low carbon manganese steel was used. At first the banding present in the initial steel was eliminated, then the two different heat treatments were applied. To reach a 0.25 volume fraction of martensite a variation of intercritical annealing temperatures was adopted for both treatments that allowed the evolution of different volume fraction of martensite. Phase analysis showed that an intercritical temperature of 725 °C (between A₃, A₁) gives the desired 0.25 V_m of martensite. A comparison of impact, tensile and ductile–brittle transition temperature (DBTT) indicates that the microstructure of the direct treatment has a better toughness. The DBTT for the DQ and SQ treatment is ?49 and ?6 °C, respectively.     

Tensile deformation of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

Two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~400–250°C) have been deformed in tension. The bainitic microstructures in these steels contain appreciable amounts of retained austenite (instead of interlath cementite), and the behaviour of this phase during tensile deformation, and its apparent influence on the mechanical properties, has been examined. In particular, it is shown that provided the retained austenite exists in an interlath, thin-film morphology it has appreciable mechanical stability. Larger volumes of retained austenite have less mechanical and thermal stability, forming plate martensite structures and also undergoing deformation twinning. The effects of these variations on tensile strength and ductility are discussed.

MST/527     

Effect of Cu addition on microstructure and impact toughness in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effects of Cu content on microstructure and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels were investigated. It has been observed that the microstructure in the simulated CGHAZ of Cu-free steel is dominated by a small proportion of acicular ferrite and predominantly bainite with martensite–austenite constituent. Whereas, in the 0.45 and 1.01% Cu-containing steels, the acicular ferrite increased significantly due to the effective nucleation on intragranular inclusions with outer layer of MnS and CuS. The formation of acicular ferrite is attributed to superior high heat-affected zone impact toughness in the 0.45% Cu-containing steel. Furthermore, the increasing martensite–austenite constituent and ε-Cu precipitates in the simulated CGHAZ of 1.01% Cu-containing steel caused degradation in impact toughness.     

Effect of continuous-cooling transformation structure on mechanical properties of 0.4C-Cr-Mo-Ni steel

Commercially available 0.4C-Cr-Mo-Ni steel was studied to determine the effects on its mechanical properties of various microstructures produced by continuous-cooling transformation after austenitization. A good combination of strength and notch toughness was obtained independently of test temperatures (293 and 193 K) when the steel was austenitized at 1173 K and then continuously cooled at an average rate of 3.1 K s^–1 (expressed as the average cooling rate from 823 to 573 K) before final rapid cooling. The microstructure of the steel consisted of a mixed structure of martensite and 10–15 vol% lower bainite, which appeared in acicular form in association with the martensite. Slower cooling had a detrimental effect on the mechanical properties of the steel; the microstructure of this steel consisted of a mixed structure of martensite and upper bainite, which appeared as masses in the matrix. As the average cooling rate increased, the lath size and internal stringer-carbide size in the upper bainite were larger, and retained a somewhat increased austenite content.     

Relationship between microstructure and hydrogen induced cracking behavior in a low alloy pipeline steel

Hydrogen induced cracking(HIC) behaviors of a high strength pipeline steel with three different microstructures, granular bainite lath bainite(GB + LB), granular bainite acicular ferrite(GB + AF), and quasi-polygonal ferrite(QF), were studied by using corrosion experiment based on standard NACE TM0284. The HIC experiment was conducted in hydrogen sulfide(H_2S)-saturated solution. The experimental results show that the steel with GB + AF and QF microstructure present excellent corrosion resistance to HIC, whereas the phases of bainite lath and martensite/austenite in LB + GB microstructure are responsible for poor corrosion resistance. Compared with ferrite phase, the bainite microstructure exhibits higher strength and crack susceptibility of HIC. The AF + GB microstructure is believed to have the best combination of mechanical properties and resistance to HIC among the designed steels.     

Modeling ductile to brittle transition temperature of functionally graded steels by fuzzy logic

In this article, a model based on fuzzy logic (FL) for predicting ductile to brittle transition temperature of functionally graded steels in both crack divider and crack arrester configurations has been presented. Functionally graded steels containing graded ferritic and austenitic regions together with bainite and martensite intermediate layers were produced by electroslag remelting. For purpose of building the model, training and testing using experimental results from 140 specimens produced from two basic composites were conducted. The used data as inputs in FL models are arranged in a format of six input parameters that cover the FGS type, the crack tip configuration, the thickness of graded ferritic region, the thickness of graded austenitic region, the distance of the notch from bainite or martensite intermediate layer, and temperature. According to these input parameters, in the FL, the ductile to brittle transition temperature of each FGS specimen was predicted. It has been found that FL model will be valid within the ranges of variables. The training and testing results in the FL model have shown a strong potential for predicting the ductile to brittle transition temperature of each FGS specimen.     

Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel

The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910 C for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 C and the remaining samples were quenched in salt bath at 425 C for various holding times. All samples were tempered at 200 C for 2 h. Microstructures were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch (CVN) impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior.     

Effect of morphology of second-phase martensite on tensile properties of Fe-0.1C dual phase steels

Fe-0.1 C steel has been studied to determine the effect of morphology (shape, size and distribution) of the second-phase martensite on the tensile properties of dual phase steels. Retention at an intercritical temperature of 1023 to 1073 K followed by ice-brine quenching (intercritical quenching treatments), whereby martensite appears to surround the ferrite grain, increased strength, but produced an increased yield ratio and decreased ductility. Incorporation of 1223 K direct ice-brine quenching prior to the intercritical quenching treatment at 1053 K gave rise to the distribution of fine spheroidized martensite in a refined ferrite matrix. The heat treatment significantly improved strength and ductility, but produced a somewhat increased yield ratio. Austenitization at a temperature of 1223 K followed by step quenching to 1023 K prior to ice-brine quenching, whereby martensite was randomly scattered in massive form in the ferrite matrix, gave a better combination of strength and ductility and produced a decreased yield ratio. These results are briefly discussed in terms of stress-strain analysis and fractography.     

The effect of martensite content on the fatigue of a dual-phase steel

Dual-phase steels having martensite contents ranging from 24 to 100% were produced by quenching sheets of a low-carbon (～0.1 weight%) vanadium-bearing steel from various temperatures in the intercritical (α + γ) and austenite phase regions. Fatigue properties of these materials were determined by constant amplitude testing of both smooth and notched specimens. It was found that, while monotonic strengths increase continuously with increasing martensite contents, cyclic strengths increase only to about 30% martensite; dual-phase steels containing less than 30% martensite cyclically harden while those with higher a martensite content soften. Similar results were obtained for strain/life behaviour, ie, improvements in life by increasing martensite content only up to 30%. In contrast to monotonic behaviour the observed variation of cyclic mechanical properties with martensite content cannot be explained by a simple rule-of-mixtures. Fatigue notch sensitivity was found to increase with increasing martensite content.     

冷却模式对热轧双相钢组织及断裂机制的影响

为研究冷却模式对热轧双相钢显微组织及断裂机制的影响,采用两段式(空冷+水冷)、连续式两种冷却方式,得到不同相比例和力学性能的热轧双相钢,轧后取样并在扫描电镜上进行原位拉伸实验.结果表明,两段式冷却模式得到的马氏体呈小岛状,而连续式冷却模式得到的马氏体呈块状,马氏体含量和形貌的不同导致两种冷却方式得到的双相钢力学性能存在差异.原位拉伸过程中,裂纹首先萌生于铁素体与夹杂物界面处,随着变形继续进行,在铁素体与马氏体界面处开始出现裂纹,当变形量进一步增大时,细小岛状马氏体始终不发生断裂,而块状马氏体在颈缩阶段发生断裂.     

Comparison of mechanical properties of martensite/ferrite and bainite/ferrite dual phase 4340 steels

Different microstructures were produced by heat treatment of 4340 steel. These microstructures are bainite, martensite, ferrite–martensite and ferrite–bainite. Mechanical tests were carried out at room temperature. The results showed that steel with bainite–ferrite microstructure has better ductility and charpy impact energy than steels with martensite–ferrite and full bainite microstructures. But yield and tensile strengths of this steel are less than the yield and tensile strengths of the other two steels. Hardness measurements showed that their hardness is the same. Fracture surface observations of tensile specimens showed increase in toughness of bainite–ferrite in comparison to martensite–ferrite and full bainite microstructures.     

Diffraction and Morphology Study of Surface Structures Accomponying with the Formation of α''''Plate(Bainite)in a Cu-Zn-AI Alloy

In the present paper the morphologies of sur-face structures and the anomalous contrast of β_1phase accompanying with the formation of bainiteplate in the Cu-Zn-Al shape memory alloy havebeen investigated in detail by the transmission elec-tron microscopy and diffraction.The experimentalresults have proved that the extra diffraction andanomalous contrast results from the surfacemartensite formed on the β_1 phase.However,thecrystal structure and morphology of the surfacemartensite are varied at different stage of bainiteformation.The observation of interfacial disloca-tion structure of bainite/β_1 matrix reveals thatthere is a large strain in the bainite transformationof β CuZnAl alloy.Based on the experiment resultsthe formation of surface martensite and bainitictransformation were discussed.     

The effect of tempering temperature on mechanical properties and microstructure of low alloy Cr and CrMo steel

Two low alloy Cr and CrMo steels with similar levels of carbon, manganese and chromium have been studied to determine the effect of tempering temperature on the mechanical properties and microstructure. The quenching and tempering of steels were carried out using a high-speed dilatometer. The steels were quenched at the average cooling rate of 30 K s-1 in the temperature range from 1123 to 573 K by flowing argon and tempered at 673, 823 and 973 K. The martensite of steels formed during quenching was of entire lath morphology with 2 vol% retained austenite. It was found that after tempering at 973 K the Cr steel contained only orthorhombic cementite, while the CrMo steel contained the cementite and hexagonal Mo2C particles in the ferrite matrix. At the same tempering conditions, the CrMo steel shows higher strength but lower ductility as compared to those of Cr steel. It is shown that this difference results from finer prior austenite grain, substructure within matrix and precipitate dispersion strengthening, primarily by Mo2C. Transmission electron microscopy (TEM) bright- and dark-field micrographs as well as selected area diffraction pattern analysis of orientation relationship showed that the cementite precipitated from the ferrite matrix. Fractography analysis showed that the morphology fracture surface was changed by increasing tempering temperature. Tempering at 973 K obtained ductile fracture by the microvoid coalescence mechanism.     

Effect of microstructure on transformation-induced plasticity of silicon-containing low-alloy steel

Fe-0.6C-1.5Si-0.8Mn steel was studied to determine the effect of the microstructure on transformation-induced plasticity (TRIP) of silicon-containing low-alloy steel. A remarkable increase in elongation through TRIP can develop in the steel subjected to the following heat treatments: (1) austemper combined with subcritical annealing (SA Aus-T): subcritical annealing at 993K followed by austempering at 673K and then light tempering (after austenitization at 1173K); (2) austemper coupled with interrupted quenching (IQ Aus-T): interrupted quenching at 533K followed by austempering at 673K and light tempering (after austenitization at 1173K).

The SA Aus-T treatment produced the triple structures of carbide-free upper bainite, retained austenite (γ_R), and free ferrite. As a result of the IQ Aus-T treatment, the triple structures of carbide-free upper bainite, γ_R, and tempered martensite appeared. The results are described and microstructural factors in TRIP are discussed.     

ULCB钢相变产物粒状与板条状贝氏体组织性能研究

对ULCB钢的主要相变组织粒状贝氏体与板条贝氏体进行了力学性能比较,结果显示粒状贝氏体与板条贝氏体是完全不同的两种组织,较高温度相变产物粒状贝氏体组织较为粗大,对强度和韧性有不利影响,而较低温度相变产物板条贝氏体组织对强度和韧性是有益的.