Corrosion fatigue characterisation of sintered multiphase stainless steels

Abstract

Multiphase stainless steels are produced for their attractive properties of mechanical strength and corrosion resistance relative to their austenite–ferrite structure. The manufacture of these steel by powder metallurgy technology presents some advantages in terms of low cost and formability of complex shapes. Mechanical and corrosion resistances are not at the level of the wrought steels due to their porous nature. In this work the fatigue and corrosion fatigue behaviour of some sintered steels obtained by sintering from 316L and 434L base powders has been studied for characterisation and comparison. The sintered steels were fatigue tested in two different environments: air and NaCl aqueous solution. The tests performed indicate that the chemical and microstructral composition has no great influence on fatigue behaviour in comparison with the manufacturing technology (sintering). This is most evident in the more aggressive environment, like seawater, in which these steels could be advantageously used. The analysis of fracture surfaces using SEM microscopy shows a peculiar crack propagation characterised by cleavage, stress intensification due to porosity, and features of localised ductility on sintering necks and base powder particles.     

Influential microstructural changes on rolling contact fatigue crack initiation in pearlitic rail steels

Abstract

Rail life is controlled by the balance between wear and fatigue damage due to in service loading. To model and optimise rail life, knowledge of the fatigue crack initiation mechanism is required. This paper reports the effect of in service loading on microstructural changes in the subsurface layer of pearlitic rail steels and observations of early stage (10–50 μm length) fatigue crack formation. Micro and nanohardness measurements are reported, along with microstructural observations, showing differential work hardening in the proeutectoid ferrite and pearlite phases. It is proposed that the differential straining results in ductility exhaustion in the proeutectoid ferrite and therefore fatigue crack initiation and initial growth in the proeutectoid ferrite phase. Observations of short (<50 μm) cracks in rails taken out of service containing significant amounts of proeutectoid ferrite (≈20%) confirm the proposed mechanism.     

Influence of thermomechanical processing and accelerated cooling on microstructures and mechanical properties of plain carbon and microalloyed steels

Abstract

A thermomechanical control process consisting of slab reheating, controlled rolling, and accelerated cooling has been adopted at the plate mill, Bhilai Steel Plant, India for achieving high strength and toughness in C–Mn and microalloyed steels while keeping the mechanical properties and the flatness constant. A mathematical model has been developed to predict the temperature distribution in the plate during accelerated cooling, taking into account the heat generation of the phase transformation. Effects of chemistry and mill parameters on ferrite grain refinement are explained in terms of nucleation and growth rate.     

Low cycle fatigue behaviour of low carbon microalloyed steel: microstructural evolution and life assessment

Abstract

In this paper the cyclic stress–strain response, low cycle fatigue (LCF) behaviour, and evolution of dislocation structures under LCF loading in the case of a low carbon microalloyed steel are discussed. The cyclic stress response revealed cyclic softening resulting from the propagation of Lüders bands. The experimental LCF life was compared with the life predicted using Tomkins' model and the modified universal slopes (MUS) equation. While the life predicted by Tomkins' model showed good correlation with the experimental results, the life predicted using the MUS equation grossly overestimated the life. Inclusion induced delaminations under cyclic loading were thought to be responsible for the overestimation by the MUS equation. Low energy dislocation structures, i.e. cells, were observed near the fracture surfaces. Interrupted tests revealed cell formation after 10 cycles at a total strain amplitude of 0·3%.     

Predicting microstructural evolution and yield strength of microalloyed hot rolled steel plate

Abstract

A mathematical model has been developed to optimise process parameters for production of API grade steel plates by thermomechanical controlled processing at a plate mill in Bhilai Steel Plant, India. The model comprises the prediction of the microstructural evolution during hot rolling, the subsequent phase transformation, and, finally, the mechanical properties of microalloyed steels. Effects of chemistry and mill parameters on recrystallisation, grain growth, and precipitation kinetics were taken into consideration to describe the metallurgical processes. . The model has been validated through laboratory experiments as well as full-scale rolling at the plate mill.     

Effects of silicon and nitrogen on hot ductility of low carbon steels

The effects of N on the hot ductility of low carbon steels have been studied with particular emphasis on the relation with Si. The ductility of Si, Al-killed steels is largely reduced by slow strain rate (10_?3–10_?4S_?1) deformation at temperatures from low temperature γ to γ/α duplex phase region (from 750 to 950 °C in this case), accompanied by ductile intergranular fracture of austenite. The cause of the loss of ductility is found to be dynamic precipitation of hexagonal close packed (hep) (Si, Al)N both on the γ grain boundaries and within the grains, and the phenomenon is largely enhanced by either increasing Si or N content. Similar phenomena, i.e. precipitation hardening-like behaviour and dependencies both of deformation conditions and of Si and N contents, are also observed in Al-free Si-killed steels. The cause of this ductility loss should be ascribed to dynamic precipitation of some kind of silicon nitride, although the precipitation has not been detected directly in all the specimens examined.     

Fabrication and characteristics of porous AISI 304L stainless steels by ceramic powder additions

Abstract

Porous AISI 304L stainless steels were fabricated by a new powder metallurgy technique, based on the addition of oxide based ceramic powders. The mixture of AISI 304L stainless steel powders and oxide based ceramic powders was compacted using a hand press at a pressure of 294 or 490 MPa. The green compacts were sintered at 1150 or 1200°C for 3 h in Ar gas atmosphere. The addition of oxide based ceramic powders into AISI 304L stainless steel powders gave rise to porous AISI 304L stainless steels with fine pores. Also, the addition of the ceramic powders increased the hardness.     

热变形对超纯净管线钢组织的影响

研究了成分和热变形对三种低碳微合金管线钢的连续冷却转变(CCT曲线)和组织的影响．结果表明：在含碳量为0．025％的低碳微合金钢中加入0．3％的Mo能推迟铁素体、珠光体转变，扩大针状铁素体(Acicular fenite)形成的冷却速度范围；高碳含量使针状铁素体向板条铁素体(Lath ferrite)转化．热变形使针状铁素体的形成温度区间从400～500℃扩大到450～700℃，显著加速相变过程，使CCT曲线明显向左上方移动，获得针状铁素体的临界冷却速度增加，抑制板条铁素体的形成，有利于获得细的针状铁素体组织，并细化岛状组织，但对残余奥氏体量影响不大。     

Influence of aluminium on carbide precipitation in low carbon microalloyed steels

Abstract

Precipitation in an 0·1C–0·5Si–1·5Mn–0·15Mo–0·5Ni–0·05V–Fe(wt-%) alloy containing from 0·04 to 0·2 wt-%Al was examined in the tempered condition. After hot rolling, the steels were solution treated at 1200°C for 2 h, then quenched in water. Tempering was mainly carried out at 600 or 650°C for 1 h. The precipitates were identified and measured using transmission electron microscopy and energy dispersive X-ray analysis. It was found that coarsening of Fe₃C carbides in the prior austenite grain boundaries was associated with low Al content, while the size of Fe₃C particles in the lath boundaries was independent of Al content. It is proposed that Al segregated to the prior austenite grain boundaries during solution treatment, associated with vacancies, and decreased the rate of vacancy migration, thereby retarding the coarsening of cementite.

MST/1707     

奥氏体变形对铌微合金钢贝氏体相变的影响

用Gleeb le-1500热模拟机研究了铌微合金钢奥氏体变形后连续冷却条件下贝氏体相变规律.研究表明,随变形温度的升高,先共析铁素体量较少,贝氏体的板条束变宽.在连续冷却条件下,贝氏体的转变量随变形量的增加而减少,随应变速率的增加而减少,但应变速率对贝氏体转变量的影响随冷却速度的增加而减弱.随着变形后保温时间的延长,奥氏体稳定性增加,在较快冷却条件下转变产物中存在残余奥氏体.     

Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 2—Plate processing

The effects of three on line accelerated cooling parameters (accelerated cooling start temperature T_A, cooling rate ?, and cooling interrupt temperature T₁) on transformation temperatures and microstructure in a low carbon microalloyed plate steel were studied by laboratory simulations in a quench deformation dilatometer. Varying the on line accelerated cooling parameters changes the austenite condition and transformation path. In general, the transformation path shifts from polygonal ferrite towards bainite with increasing T_A, increasing ?, and decreasing T₁. There is also a corresponding refinement in the microstructure and increase in hardness. In comparison with the laboratory thermomechanical processing treatments, the multipass industrial rolling schedule produces a much more heavily deformed austenite structure than laboratory thermomechanical processing treatments, which would favour high transformation temperatures, fine polygonal ferrite microstructure, and lower hardness.

MST/3425     

Effects of deformation rate and cooling conditions on low temperature plasticity of stainless iron-chromium-nickel steels

The thermal model described previously (CRYOGENICS, Vol 16 No 9 (1976) 533) is used to explain experimental results obtained by measuring the effect of deformation rate and cooling conditions on low temperature plastic strain. The samples used were polycrystalline phase-transition stable Fe-Cr-Ni alloy steels.     

Effects of hot deformation on austenite transformation in 10w carbon Mo-Nb and C-Mn steels

Abstract

The 0.15C-1.5Mn and 0.07C-0.3Mo-0.055Nb steels were subjected to hot rolling to determine the effects on transformation to ferrite and to hot torsion to measure strength. After 25 or 50% reduction at 830°C, the acceleration during isothermal transformation in the range 700–600°C is much greater for C-Mn than for Mo-Nb; delays of 5–10 s before cooling reduce the acceleration markedly. In continuous cooling, three passes of 20% reduction raise the temperature of austenite decomposition, most noticeably at the higher rate 9 K S^-l. At this rate, for the C-Mn the pearlite is refined and for Mo-Nb the promoted ferrite is refined more and enhanced over bainite than at lower rates. Intercritical rolling below Ar₃ enhanced ferrite formation more in C-Mn in the range 780–720°C than in Mo-Nb over 750–690°C. The stronger Mo-Nb in torsion (900–1100°C, 0.1-5 S^-l) exhibits an activation energy of 353 kJ mol^-l compared with 316 kJ mol^-l for C-Mn.     

Precipitation and hot ductility of low C-V and low C-V-Nb microalloyed steels during thin slab casting

Abstract

The as cast hot ductility of low C-V, low C-V-Nb, and niobium microalloyed steels has been investigated using in situ melted tensile specimens, which were subjected to cooling rates and strain rates found typically in thin slab casting. Stress relaxation tests were performed on in situ melted specimens to monitor the kinetics of strain induced precipitation in the above steels. Although the addition of niobium to low C-V and low C-V-N steels increased the temperature at which ductility began to deteriorate, ductility was improved in the low temperature region of the austenite. This was attributed to a delay in NbVC,N precipitation in V-Nb steels. Increasing the nitrogen content did not influence the temperature at the onset of ductility loss in low C-V and low C-V-Nb steels, but did increase the temperature marking the onset of the ductility trough. Faster cooling rates led to a deterioration of ductility in low C-V-N and low C-V-Nb-N steels. The as cast ductility of low C-V and low C-V-Nb steels was superior to that as of cast peritectic C-Nb steel as well as reheated peritectic C-V and peritectic C-V-Nb steels.     

Precipitation in V bearing microalloyed steel containing low concentrations of Ti and Nb

Abstract

The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0·007–0·008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V, Nb, Ti)(C, N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45–70 nm), spherical (20–45 nm), irregular (20–45 nm), and fine (10–20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite–ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti, Nb, V)N) in the core and carbides ((Ti, Nb, V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti_0·53Nb_0·35V_0·12 and Ti_0·6V_0·4, Nb_0·51V_0·49 and Ti_0·64Nb_0·36. Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1₂ structure and exhibited a Baker–Nutting orientation relationship with the ferrite matrix. M₄C₃ types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.     

Constitutive modelling of carbon and alloy steels

Abstract

Semi-empirical models for the constitutive behaviour of steels often fail to predict the flow stress with sufficient accuracy. A simple neural network structure 3 : 4 : 1 is able to model flow behaviour better than other models available in the literature. It has been developed for four carbon steels, two microalloyed steels, an austenitic stainless steel and a high speed steel.     

Relative importance of transformation temperatures and sulphur content on hot ductility of steels

Abstract

Low (0·3%) and high manganese (1·4%) plain C – Mn steels with varying sulphur levels have had their hot ductility determined over the temperature range 700 – 1000°C, both after 'solution treatment' at 1330°C and directly after casting. It has been established that the width, depth and position of the hot ductility curves after solution treatment is more related to the transformation behaviour than either the sulphur in solution or the sulphide volume fraction or distribution. The growth of deformation induced ferrite at the austenite boundaries seems to be mainly diffusion controlled, and the higher is the transformation temperature for the γ – α phase change, the faster is the growth. Large amounts of ferrite can then form, giving good ductility. Thus, high transformation temperatures Ae ₃ or Ar ₃ are required to produce narrow ductility troughs. It is believed that any detrimental influence of the sulphides on these 'solution treated' steels is swamped by the rapid increase in ferrite volume fraction. For the as cast state, as more sulphides are able to precipitate at the interdendritic boundaries and austenite grain boundaries than in the solution treated condition, increasing the sulphur level causes a small deterioration in ductility at the high temperature end of the trough. In the present work, only narrow troughs have been found. This is in contrast to previous work on as cast C – Mn – Nb – Al steels, which exhibited wide troughs in the ductility curves, where it was shown that higher total sulphur levels lead to considerably worse ductility and that sulphur can be as detrimental to the ductility as niobium. It is recommended that, to avoid transverse cracking during continuous casting, in addition to keeping the sulphur level low, the carbon and manganese should also be as low as possible.     

Influence of the chemical composition on transformation behaviour of low carbon microalloyed steels

In order to design thermomechanical schedules for processing low carbon microalloyed steels, the various critical transformation temperatures, i.e. the start and finish of the austenite transformation (A_r3, A_r1) and the non-recrystallization temperature (T_nr), must be determined. Continuous cooling torsion and compression testing are useful ways to measure these values. In this study six low carbon microalloyed steels with different additions (Nb, Cu, Si and Mo) were examined using these techniques. Moreover, the equilibrium phase diagrams for each alloy were calculated using FactSage. The comparison of the thermomechanical testing results with the thermodynamic calculations leads to a better understanding of the effect of the different elements on the transformation behaviour of pipeline steels. Regarding transformation temperatures, Cu in residual contents showed a strong effect on decreasing both A_r3 and A_r1, which indicates a hardenability effect of this element. On the other hand, increasing Nb contents increased T_nr by accelerating Nb(C,N) precipitation. However, when Si was added to a Nb-microalloyed steel, the T_nr decreased.