A new etching route for revealing the austenite grain boundaries in an 11.4% Cr precipitation hardening semi-austenitic stainless steel

A detailed metallographic characterization of a precipitation hardening semi-austenitic stainless steel is described. A new etching procedure based on the Lichtenegger and Blöch color etching solution, which is frequently used in duplex stainless steels to differentiate delta ferrite from austenite, has been used to differentiate martensite, austenite and the Chi-phase in this stainless steel. By changing the etching conditions, this etchant now reveals the austenite grain boundaries when the steel is in the austenitic state. Moreover, this solution is able to reveal also the prior austenite grain boundaries when the steel is in its martensitic state. This etching procedure represents a great advantage because it reveals, at the same time, different features of the microstructure.     

Investigation of dual phase transformation of commercial low alloy steels: Effect of holding time at low inter-critical annealing temperatures

Dual phase steels are a class of steels characterized by a microstructure consisting of a soft ferrite matrix with hard martensite islands at the grain boundaries. The temperature in the dual phase (α + γ) region and the holding time are two important parameters in the intercritically annealing process. In this study, different grades of commercial low alloy steels have been heat treated to a constant annealing temperature by changing the holding time. It is observed that the effect of holding time is dependent on the steel composition. In this context, a microstructural examination has been carried out using optical, scanning electron microscope and electron probe micro-analysis and hardness values have been determined.     

Effect of rare earths on impact toughness of a low-carbon steel

Studies of an industrial low-carbon steel (B450NbRE) suggest that the impact toughness is unexpectedly low under its practical service, probably resulting from the unstable recovery of rare earths (RE) in steelmaking. The purpose of this work is to investigate the effect of RE on the impact toughness in low-carbon steel. The B450NbRE steels with content of 0.0012–0.0180 wt.% RE were produced by vacuum induction furnace. The impact toughness and microstructure were investigated after hot rolled. The Gleeble-1500 thermal simulator was used to validate the effect of RE on the microstructure. The results indicate that the microstructure of hot-rolled steels is characterized by polygonal ferrite, quasi-polygonal ferrite, bainite and pearlite. The impact toughness increases with RE contents reaching the peak with content of 0.0047 wt.% RE, such a change exhibits the same rule as the case of the ferrite amount. However, this improvement in impact toughness is not only due to an increase in ferrite amount, but also the fine grained structure and the cleaner grain boundaries. And content of 0.0180 wt.% RE is excessive. Such an addition of the RE resulted in the martensite precipitates at the grain boundaries, which are extremely detrimental to impact toughness.     

Mechanical properties of ultrafine grained ferritic steel sheets fabricated by rolling and annealing of duplex microstructure

A new route to fabricate ultrafine grained (UFG) ferritic steel sheets without severe plastic deformation is proposed in this article. A low-carbon steel sheet with a duplex microstructure composed of ferrite and martensite was cold-rolled to a reduction of 91% in thickness, and then annealed at 620–700 °C. The microstructure obtained through the process with annealing temperatures below 700 °C was the UFG ferrite including fine cementite particles homogenously dispersed. The grain size of ferrite matrix changed from 0.49 to 1.0 μm depending on the annealing temperature. Dynamic tensile properties of the produced UFG steels were investigated. The obtained UFG ferrite–cementite steels without martensite phase showed high strain rate sensitivity in flow stress. The UFG ferritic steels are expected to have high potential to absorb crash energy when applied to automobile body.     

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.     

Effects of heat input on microstructure and mechanical properties of Fe–2Cr–Mo–0.12C steel

ABSTRACT

Thermal simulated specimens with the heat inputs of 20, 50 and 80?kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe–2Cr–Mo–0.12C pressure-vessel steel. The results indicated that the microstructures in the coarse-grained heat affected zone of tested steels with various heat inputs were mainly consisted of lath martensite and bainite ferrite. As the heat input increased, the fraction of martensite decreased and the bainite ferrite fraction increased. The toughness (tested at ?40°C) and hardness for the heat input of 50?kJ/cm were 102?J and 346?HV, respectively, which was attributed to the high-volume fraction (60%) of the high-angle grain-boundary and the fine bainite lath.

This paper is part of a thematic issue on Nuclear Materials.     

Grain boundary engineering of titanium-stabilized 321 austenitic stainless steel

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent intergranular corrosion due to sensitization in unstabilized 304 and 316 austenitic stainless steels, but the effect of GBE on intergranular corrosion in stabilized austenitic stainless steels has not been clarified. In this study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into a titanium-stabilized 321 austenitic stainless steel. The resulting steel showed much higher resistance to intergranular corrosion after sensitization subsequent to carbon re-dissolution heat treatment during the ferric sulfate–sulfuric acid test than the as-received one. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular corrosion during GBE.     

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.     

正火型V-N-Ti和Nb-V-Ti海工钢焊接粗晶热影响组织和韧性研究

用焊接热模拟方法研究了V-N-Ti和Nb-V-Ti微合金化正火型海工钢模拟粗晶热影响区(CGHAZ)组织和韧性的变化规律。结果表明,组织的不同使V-N-Ti设计正火型海工钢的模拟CGHAZ韧性比Nb-V-Ti钢的好。对于V-N-Ti钢,较高的N含量提高了富Ti(Ti, V)(C, N)粒子析出温度和铁素体形核能力,使模拟CGHAZ原始奥氏体晶粒和(取向差角为15°)晶粒细化,并生成能阻止或使解理裂纹的偏转细小多边形铁素体,因此具有良好的低温韧性。而Nb-V-Ti钢模拟CGHAZ原奥氏体晶界上的链状M-A、粗大的原始奥氏体晶粒和有效晶粒尺寸,是模拟CGHAZ韧性差的原因。     

Microstructure of X52 and X65 pipeline steels

The microstructure of two commercial pipeline steels X52 and X65 was examined to provide a foundation for the understanding of the IGSCC mechanism of pipeline steels. Observation of the microstructure was carried out using scanning electron microscopy (SEM) and an analytical electron microscope. The microstructure of X52 and X65 pipeline steels shows banding of pearlite rich and ferrite rich areas. The ferrite grains were about 10 μm in size with curved grain boundaries. There was carbide at the ferrite grain boundaries for X52 steel, and there was circumstantial evidence to suggest carbon segregation at the boundaries. The pearlite colonies were consistent with nucleation by a number of different mechanisms. This revised version was published online in September 2006 with corrections to the Cover Date.     

Failure mechanisms in rephosphorised sheet steel spotwelds

Abstract

The influence of phosphorus and sulphur on the microstructures and cross-tension strength of spotwelds in sheet steels has been studied. In steels containing 0·06%C and 0·33%Mn (wt-%), the addition of phosphorus had two effects on the microstructure. An addition of 0·10%P increased the hardenability, resulting in a martensitic weld and heat affected zone. A further increase to 0.15%P resulted in the stabilisation of ferrite in the weld and fusion zones. The ferrite along grain boundaries in the fusion zone aided the initiation of cleavage and decreased the cross-tension strength. The addition of sulphur to a steel containing 0·15%P decreased the cross-tension strength. This effect was associated with a change from ductile to brittle behaviour at the interface between the sheets in the heat affected zone, caused by increased wetting of the interface and grain boundaries by oxysulphides as the sulphur content was increased.

MST/920     

热输入对10CrNi3MoV钢MAG焊接头组织和性能的影响

为制定合理的焊接工艺,保证焊接质量,设置不同焊接热输入进行了10CrNi3MoV钢MAG焊接。采用微观组织分析、断口观察、力学测试等手段研究了焊接热输入对接头组织及性能的影响。结果表明,热输入较小时（E=11.0 kJ·cm^-1、E=14.4 kJ·cm^-1）,焊缝组织以针状铁素体为主,并含有部分粒状贝氏体、先共析铁素体等;热输入较大时（E=18.1 kJ·cm^-1）,针状铁素体占比降低,粒状贝氏体、先共析铁素体等增多,组织粗化。随热输入的增大,粗晶区晶粒粗化,组织由板条马氏体逐步转变为板条贝氏体,板条界限模糊,并有粒状贝氏体出现;焊缝金属强度降低,冲击韧性先略有升高后显著降低,断裂形式由微孔聚缩型韧断变为准解理/韧性混合断裂。热输入E=14.4 kJ·cm^-1时,焊缝组织以细密的针状铁素体为主,具有最佳强韧性匹配。     

Effect of grain boundary precipitation and δ-ferrite formation on surface defect of low nickel austenitic stainless steels

The present study elaborately discussed the effect of grain boundary precipitation and delta (δ) ferrite formation on surface defects of low nickel austenitic stainless steels. Several specimens were collected from different locations (i.e. centre and side) of cast slabs and rolled plates with variable nitrogen and carbon contents. The alloy segregation, precipitation and δ-ferrite formations were analyzed using several characterization tools. It was observed that the segregation of alloying elements along the grain boundaries is solely responsible for variation in precipitation and δ-ferrite formation among the specimens. Type of precipitation is mainly composition dependant. Relatively higher nitrogen and low carbon containing specimens show Cr₂N and/or Cr₃C_1.52N_0.48 rich continuous precipitation along the grain boundaries. On the other hand, decrease in nitrogen content along with the increase in carbon content results in discontinuous or cellular type of precipitation. Continuous grain boundary precipitates were found more detrimental compare to discontinuous or cellular precipitations. Micro-cracks are probably generated at the stress concentrated areas of precipitation–matrix interfaces and propagate along the interfaces or grain boundaries towards the stress free surface. Again, additional driving forces towards the surface crack formation and propagation are facilitated by the δ-ferrite and austenite interfaces during hot rolling.     

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.     

Influence of cementite spheroidization on relieving the micro-galvanic effect of ferrite-pearlite steel in acidic chloride environment

The corrosion behavior of the as-received steel and the spheroidized steel in acidic chloride environment was investigated. The results indicate the corrosion mode and corrosion rate of two steels are diverse due to their difference in microstructure. For as-received steel with ferrite-pearlite microstructure, severe localized corrosion happens on the pearlite regions, and plenty of cathodic cementite remains in the pits, further strengthening the micro-galvanic effect and accelerating the corrosion rate. While for spheroidized steel with tempered martensite microstructure, the nanosized cementite particles evenly distributed on the ferrite substrate are easy to fall off, which can significantly reduce the cementite accumulation on the steel surface, relieving the acceleration effect of micro-galvanic corrosion.     

铁素体-珠光体型非调质钢的高周疲劳破坏行为

研究了三种碳和钒含量不同的铁素体-珠光型非调质钢的高周疲劳破坏行为,并与调质钢进行了对比.结果表明,铁素体-珠光体型非调质钢的高周疲劳性能与其微观组织特征有关.提高铁素体相硬度,其疲劳极限及疲劳极限比均提高,疲劳极限比最高可达0.60,远高于调质钢的0.50;热轧态粗大的网状铁素体-珠光体组织的疲劳性能较差,低于同等强度水平的高温回火马氏体组织。铁素体-珠光体型非调质钢疲劳破坏机制不同于调质钢,其疲劳裂纹基本上萌生于试样表面的铁素体/珠光体边界,并优先沿着铁素体/珠光体边界扩展;对于同等强度水平的调质钢,不存在像铁素体那样的软相,因而易在试样表层粗大的夹杂物处萌生疲劳裂纹.     

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.     

MICROSTRUCTURAL AND ENVIRONMENTAL EFFECTS ON FATIGUE CRACK PROPAGATION IN DUPLEX STAINLESS STEELS

Abstract— Fatigue crack initiation and propagation in duplex stainless steels are strongly affected by microstructure in both inert and aggressive environments. Fatigue crack growth rates in wrought Zeron 100 duplex stainless steel in air were found to vary with orientation depending on the frequency of crack tip retardation at ferrite/austenite grain boundaries. Fatigue crack propagation rates in 3.5% NaCl solution and high purity water are increased by hydrogen assisted transgranular cyclic cleavage of the ferrite. The corrosion fatigue results are interpreted using a model for the cyclic cleavage mechanism.     

热输入对Q1100钢焊接接头低温韧性及耐蚀性能的影响

采用10 kJ/cm和15 kJ/cm两种焊接热输入对Q1100超高强钢进行熔化极气体保护焊,研究焊接接头的组织性能及局部腐蚀行为。结果表明：两种热输入焊接接头的焊缝组织主要为针状铁素体和少量的粒状贝氏体,粗晶区组织均为板条贝氏体,细晶区组织均为板条贝氏体和粒状贝氏体,临界相变区组织为多边形铁素体、马奥岛和碳化物的混合组织。两种热输入焊接接头中电荷转移电阻均为母材>热影响区>焊缝区,母材耐蚀性最好,热影响区次之,焊缝区耐蚀性最差。在腐蚀过程中,焊缝区作为阳极最先被腐蚀,当腐蚀一定时间后,腐蚀位置发生改变,阳极腐蚀区域转移到母材区,而焊缝区作为阴极得到保护。热输入为10 kJ/cm时,焊接接头具有更好的低温韧性和耐蚀性,其焊缝和热影响区-40℃冲击功分别为46.5 J和30.2 J。     

Fatigue performance of dual‐phase steels for automotive wheel application

Fatigue performance of ferrite–martensite (FM) and ferrite–bainite (FB) dual‐phase (DP) steels used in automotive wheels has been compared in terms of (i) high‐cycle fatigue performance and failure mechanisms and (b) low‐cycle fatigue performance (Δε_t/2 = 0.002 to 0.01) and associated deformation mechanisms. FBDP steel exhibits moderately better high‐cycle fatigue performance, owing to delay in microcrack initiation. In FBDP steel, microcracks initiate predominantly along ferrite grain boundaries, while that at FB interface is significantly delayed in comparison with FMDP steel, where few microcracks appear at FM interface even below the endurance limit. During low‐cycle fatigue, however, FMDP steel performs considerably better than FBDP steel till Δε_t/2 ≤ 0.005 attributed to initial cyclic hardening, followed by cyclically stable behaviour exhibited by FMDP steel. In sharp contrast, at all Δε_t/2 > 0.002, FBDP steel undergoes continuous cyclic softening. The latter may cause undesirable deformation of wheels in service.