Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

Influence of manganese and sulphur on overheating of some low-alloy steels

Abstract

An assessment has been made of the overheating behaviour of three low-alloy steels used in the electric power generating industries. The steels, 1Cr–Mo–V, 2·25Cr–1Mo, and 3·5Ni–Cr–Mo–V have been prepared as high-purity versions with low tramp element contents, sulphur contents of 0·001%, and manganese contents of 0·02 and 0·2%. For comparison, commercial steels produced by good practice and containing 0·006–0·011%S and 0·17–0·21%Mn have also been examined (all compositions in wt-%). The upper shelf energies of the high-purity versions of the steels in the fully heat treated condition indicate that these steels do not overheat after treatment at temperatures up to 1400°C, whereas the commercial versions do overheat and, in some cases, show a severe reduction in their impact energy levels. In some cases, the high-purity steels show an unusually low tendency to austenite grain growth after reheating at temperatures up to 1400°C. The results obtained show that new specifications for low–alloy steels could be developed which would give freedom from overheating during forging and greatly improved upper shelf energies after heat treatment.

MST/362     

Combined equilibrium and non-equilibrium segregation treatment of temper embrittlement in low alloy steels

Abstract

Embrittlement is an important factor for low alloy ferritic steels used for components and structures in the power and petrochemical industries when exposed to a higher temperature. The embrittlement may be classified into non-hardening embrittlement and hardening embrittlement. The non-hardening embrittlement, for example temper embrittlement, originates from grain boundary segregation of impurity elements such as phosphorus. To predict this segregation behaviour, a model is established by simplifying a low alloy steel as a dilute Fe–C–Mo–P quaternary alloy and modifying previous models. This model is applied to segregation predictions in a 2.25Cr–1Mo steel subjected to a complex heat treatment cycle.     

压铸模的失效分析与失效抗力指标

对铝、铜合金压铸模具进行失效分析和对国内同类模具进行失效类型统计;实测３Ｃｒ２Ｗ８Ｖ,４Ｃｒ５Ｍｏ２ＭｎＶＳｉ和４Ｃｒ３Ｍｏ２ＭｎＶＮｂＢ钢的六种性能,找出了影响铝、铜压铸模使用寿命的主要失效抗力指标。将三种钢制作同种模具进行实物考核,证实符合抗力指标要求的热模钢使用寿命长。     

Predicting damage and failure under low cycle fatigue in a 9Cr steel

Experimental data have been generated and finite element models developed to examine the low cycle fatigue (LCF) life of a 9Cr (FB2) steel. A novel approach, employing a local ductile damage initiation and failure model, using the hysteresis total stress–strain energy concept combined with element removal, has been employed to predict the failure in the experimental tests. The 9Cr steel was found to exhibit both cyclic softening and nonlinear kinematic hardening behaviour. The finite element analysis of the material's cyclic loading was based on a nonlinear kinematic hardening criterion using the Chaboche constitutive equations. The models’ parameters were calibrated using the experimental test data available. The cyclic softening model in conjunction with the progressive damage evolution model successfully predicted the deformation behaviour and failure times of the experimental tests for the 9Cr steels performed.     

Damage accumulation modeling under uniaxial low cycle fatigue at elevated temperatures

The paper presents a fatigue damage accumulation model, which allows us to predict fatigue life under low cycle uniaxial loadings at elevated temperatures. The structure of the model has been based on the stress–strain curves obtained during the experimental study. The model has been verified experimentally by applying experimental studies carried out on ENAW-2024T3 aluminum alloy and 2Cr–2WVTa steel. Moreover, a comparison between the results of fatigue life prediction using the proposed damage accumulation model was done with the results obtained on the basis of various generally applied models, based on the Manson–Coffin dependency. Furthermore this paper presents the results of experimental studies carried out on the aluminum alloy ENAW 2024 T3 under uniaxial low cycle fatigue loadings in the conditions of elevated temperatures. In the course of the study, material constants and the parameters of the stress–strain curve in the range of low cycle fatigue for four levels of temperatures (20, 100, 200 and 300 °C) were set.     

A comparative study of the fretting fatigue behavior of 4340 steel and PH 13-8 Mo stainless steel

The fretting fatigue behavior of two high strength structural steels, PH 13-8 Mo stainless steel and quenched and tempered 4340 steel, is investigated. Both were heat treated to a similar hardness (43-44 HRC), comparable to the condition used in structural components. Both materials experienced significant reductions in fatigue strength due to fretting, with PH 13-8 Mo stainless steel exhibiting a greater susceptibility to fretting than 4340 steel, when operating in the mixed fretting regime. The use of fretting pads with different surface profiles showed that contact geometry did not significantly influence the fretting fatigue behavior of either steel for the range of loading conditions considered. The fretting fatigue lives are discussed in light of the low cycle fatigue and crack growth rate behavior of these steels. The life trends in fretting fatigue correlate more closely to the low cycle fatigue behavior.     

Development of carbon—Low alloy steel grades for low temperature applications

Low alloy steels are processed to fulfill the requirements of low temperature applications. Besides the chemical composition, the steel should receive a suitable heat treatment to ensure the targeted mechanical properties at low temperature. In other words, the steels are designed to delay the ductile to brittle transition temperature to resist dynamic loading at subzero temperatures. Steel alloys processed for liquefied gas pipeline fittings are examples for applications that need deep subzero impact transition temperature (ITT).The main purpose of the present work was to find a suitable heat treatment sequence for alloys LC2 and LC2-1. Further, it aimed to correlate the impact toughness with the microstructure and the fracture surface at different sub-zero temperatures.The steels under investigation are carbon-low alloy grades alloyed with Ni, Cr and Mo. LC2 steel alloy has been successfully processed and then modified to LC2-1 alloy by addition of Cr and Mo. Oil quenching from 900 °C followed by tempering at 595 °C was used for toughness improvements. Hardness, tensile and impact tests at room temperature have been carried out. Further impact tests at subzero temperatures were conducted to characterize alloys behavior. Metallographic as well as SEM fractographic coupled with XRD qualitative analysis are also carried out.Non-homogenous martensite-ferrite cast structure in LC2 was altered to homogeneous tempered martensite structure using quenching-tempering treatment, which is leading to shift the ITT down to −73 °C. Addition of Cr and Mo creates a very fine martensitic structure in LC2-1 alloy. Quenching-tempering of LC2-1 accelerates ITT to −30 °C. It is expected that the steel was subjected to temper embrittlement as a result of phosphorus segregation on the grain boundary due to Cr and Mo alloying, as it was concluded in reference no. [6].     

Base material fatigue data for low alloy forged steels used in the subsea industry. Part 2: Effect of cathodic protection

In air S–N fatigue data for forged low alloy steels as used in the subsea industry are presented in Part 1 of this paper. The test scope in Part 1 included testing to quantify the effect of the surface roughness, mean stress and material strength on the high cycle fatigue strength of low alloy steels with a tensile strength in the range of 600–800 MPa. A method for estimating the in air S–N curve from the tensile strength (material grade), surface roughness (machining) and mean stress (such as residual stresses, pressure testing, pre-load and external loads) is presented in Part 1. In this Part 2, fatigue test results for low alloy steels and one carbon steel tested in seawater with cathodic protection with a potential of −1050 mV versus an Ag/AgCl reference electrode are presented. The fatigue testing has been performed using smooth specimens. The tested smooth specimens have (actual) tensile strengths in the range from 627 to 790 MPa. Penalty factors for the tested smooth specimens in seawater with cathodic protection with respect to in air performance (Part 1) are presented and compared with penalty factors used in fatigue design codes such as DNVGL-RP-0005 (former DNV-RP-C203) and BS 7608. The obtained environmental reduction factors are found to be in accordance with the penalty factors used in BS 7608 provided that the maximum stress in the cycle is less than 94% of the yield stress for the material. The penalty factors used for forged steels in DNVGL-RP-0005 are non-conservative compared to the test outcome for the steel tested in an artificial 3.5% NaCl seawater solution. For higher stress levels, larger penalty factors than used in BS 7608 are required. It is found that the obtained S–N based environmental reduction factors are of similar magnitude as BS 7910 fatigue crack growth based reduction factors for CP.     

考虑应变路径的多轴低周疲劳寿命预测模型

通过分析材料在多轴非比例加载下产生附加强化的机理,该文以拉扭薄壁管试件为研究对象,分析了临界平面上的应变状态,并在此基础上以塑性应变能为控制参数定义表征多轴低周疲劳寿命对应变路径依赖性的非比例度。基于多轴疲劳临界损伤面原理,应用von-Mises 准则和本文定义的应变路径非比例度参数建立起能反映应变路径对非比例附加强化影响的多轴低周疲劳寿命预测模型。利用该模型预测08X18H10T 不锈钢、Ti-6Al-4V合金、S460N 钢和2.25Cr-1Mo 钢这4 种材料的多轴疲劳寿命,并与试验值进行比较。结果表明:该模型的预测结果与试验结果吻合良好,能同时适用于比例与非比例加载,预测精度较高,便于工程应用。     

金属材料裂纹扩展Paris律的模拟方法与应用

结合TA12、TC4钛合金和Cr2Ni2MoV钢的Paris律实验结果,对基于材料的低周疲劳临界损伤获取材料疲劳裂纹扩展Paris律的有限元模拟方法(称为LFF方法)进行了有效性验证,并开展了拓展应用。实验结果与模拟结果比较表明,LFF方法用于模拟材料Paris律有良好准确度。应用LFF方法获得了Cr2Ni2MoV钢、N18合金在多种高温下的Paris模型参数,研究了疲劳裂纹扩展速率的温度效应。     

Influence of microstructure and load ratio on cyclic fatigue and final fracture behavior of two high strength steels

The results of a recent study aimed at understanding the conjoint influence of load ratio and microstructure on the high cycle fatigue properties and resultant fracture behavior of two high strength alloy steels is presented and discussed. Both the chosen alloy steels, i.e., AerMet® 100 and PremoMet™ 290 have much better strength and ductility properties to offer in comparison with the other competing high strength steels having near similar chemical composition. Test specimens were precision machined from the as-provided stock of each steel. The machined specimens were deformed in both uniaxial tension and cyclic fatigue under conditions of stress control. The test specimens of each alloy steel were cyclically deformed over a range of maximum stress at two different load ratios and the number of cycles to failure recorded. The specific influence of load ratio on cyclic fatigue life is presented and discussed keeping in mind the maximum stress used during cyclic deformation. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic mode and to concurrently characterize the intrinsic features on the fracture surface. The conjoint influence of nature of loading, maximum stress and microstructure on cyclic fatigue life is discussed.     

低合金钢烧结硬化性能研究

研究了4种成分的低合金钢材料烧结硬化性能。结果表明,添加Mo、Mn、Cr等合金元素的烧结低合金钢抗拉强度、硬度明显提高,合金强化效果显著。采用烧结硬化工艺,低合金钢都获得了比常规烧结更好的力学性能。其中硬化能力倍数高的4#材料效果最佳,硬度达到HRC35.4,强度为540.74 MPa,接近常规热处理的性能。金相分析表明,烧结硬化组织主要是马氏体,此外有少量残留奥氏体、铁素体、珠光体。     

A new ferritic stainless steel with improved thermo-mechanical fatigue resistance for exhaust parts

Abstract

Over the past few years, car manufacturers have been considering ever higher service temperatures for the engine in order to comply with the constraints of depollution standards. The requirements in terms of exhaust gas temperature could easily reach and overtake the limits of common stainless steel grades used for such applications in the coming years.

A new ferritic stainless steel – named K44X – with increased high temperature resistance has therefore been developed to withstand service temperature up to 1000 °C. K44X belongs to EN 1.4521 and AISI 444 classifications and is composed of approximately 19% Cr, 2%Mo and 0.6% Nb. This specific composition leads to better mechanical properties, higher creep and fatigue resistance than EN 1.4509, while keeping comparable weldability and formability. Its coefficient of thermal expansion is lower in comparison to austenitic stainless steel grades and its resistance to cyclic oxidation is improved significantly.

High-temperature properties (mechanical properties, creep, cyclic oxidation resistance, and high cycle fatigue) of K44X are presented in this paper and compared with common ferritic and austenitic stainless steels used in the hot end of exhaust lines. A thermal fatigue test – designed to reproduce exhaust manifold service conditions – has also been carried out with the highest temperatures of the cycle in the range of 850–1000 °C. The results of these thermal fatigue tests were compared with the above-mentioned stainless steels. A thermal fatigue damage criterion was then identified based on these experimental results and using a cyclic behaviour law obtained from isothermal low cycle fatigue tests.     

Differential scanning calorimetry study of diffusional and martensitic phase transformations in some 9 wt-%Cr low carbon ferritic steels

Abstract

The results of a comprehensive characterisation study of different phase transformations that take place upon heating and cooling in some low carbon, 9 wt-%Cr steels with varying concentrations of microalloying additions are presented in this paper. The steels investigated include: standard 9Cr–1Mo grade, V and Nb added modified 9Cr variety, controlled silicon added versions of plain 9Cr variety, (Ni+Mn) content controlled modified 9Cr welding consumables and one composition of W, Ta added reduced activation steel. The various on-heating diffusional phase changes up to the melting range and subsequent rapid cooling induced martensitic transformations are investigated in a controlled manner using differential scanning calorimetry under different heating and cooling rates, in the range 1–100 K min^?1. In addition to the accurate determination of Ac₁, Ac₃, M₂₃C₆, MX carbide dissolution and δ-ferrite formation temperatures upon heating, the melting range and the associated fusion enthalpy have also been established for these steels. The effect of prolonged thermal aging at temperatures of 823–873 K on austenite formation characteristics has also been investigated for standard and modified 9Cr–1Mo steels. The critical cooling rate for the formation of martensite on cooling from single phase austenite region is estimated to be about 4–5 K min^?1 for all 9Cr steels investigated in this study. The effect of holding at 1273 K in the austenite region on martensite start temperature M_s, has also been evaluated as a part of this study. The experimental results are discussed in the light of the prevailing understanding of the physical metallurgy of high chromium low carbon steels.     

Zum thèrmisch-mechanischen Ermüdungsverhalten von NiCr22Co12Mo9

On the Thermal-Mechanical Fatigue Behaviour of NiCr22Co12Mo9 The fatigue behaviour of the Ni-based alloy NiCr22Co12Mo9 (corresponding to Inconel 617) under combined cyclic thermal and mechanical “in-phase”- and “out-of-phase”-loading was investigated with a constant minimum cycle temperature of 473K and a constant total strain amplitude of 6,25% at maximum cycle temperatures T_o ranging from 873K to 1473K. It was found that the cyclic deformation behaviour and the corresponding development of the microstructure during the tests were mainly determined by the maximum cycle temperatures. With increasing T_o increasing recovery processes occurred accompanied by charakteristic changes in the microstructure which reduced cyclic hardening. In contrast, both maximum cycle temperature and cycle mode determined surface deteriorations, which were characterized by surface cracks, and fatigue life. At the highest temperatures during the in-phase-loading cycles, the occuring tensile stresses caused increasing amounts of intergranular damage with corresponding reductions of fatigue life.     

Effect of toughness on low cycle fatigue behavior of pipeline steels

The effect of toughness on the fatigue behavior of pipeline steels was investigated, including the fatigue crack propagation rate and low cycle fatigue test under the loading condition simulating the actual operation of pipelines. The results indicate that the toughness can strongly influence the fatigue behavior of pipeline steels (i.e., the steels with high toughness possess high resistance to fatigue crack propagation and high tolerance of damage, which are much beneficial to obtaining a long life for line pipe structures).     

Simulation of elevated temperature fatigue damage evolution using the finite element method for near alpha titanium alloy

The numerical estimation of evolving damage under low cycle fatigue loading condition has been performed in the near‐α titanium alloy IMI‐834 at 823 K temperature. By using the experimentally determined parameters as input, numerical simulation of fatigue damage has been performed on round specimens using finite element analysis. Coupled deformation‐damage model has been established for this alloy for simulation of damage evolution in a three‐dimensional cylindrical low cycle fatigue test specimen. The fatigue damage estimates from numerical simulation are observed to be in close agreement with the experimental results.     

An assessment of the role of near-threshold crack growth in high-cycle-fatigue life prediction of aerospace titanium alloys under turbine engine spectra

Increasingly accurate life prediction models are required to utilize the full capability of current and future advanced materials in gas turbine engines. Of particular recent interest are predictions of the lifetimes of engine airfoil materials that experience significant intervals of high-frequency, high-cycle fatigue (HCF). Conventional life management practices for HCF in the turbine engine industry have been based principally on a total-life approach. There is a growing need to develop damage tolerance methods capable of predicting the evolution and growth of HCF damage in the presence of foreign object damage (FOD), low cycle fatigue (LCF), and surface fretting fatigue. To help identify key aspects of the HCF life prediction problem for turbine engine components, a review is pressented of the extensive results of an Air Force research contract with Pratt & Whitney on the high strength titanium alloy Ti-8Al-1Mo-1V. Data from this representative turbine-airfoil material are used to examine the applicability of linear elastic fracture mechanics methods for prediction of service lifetimes under load spectra that include high cycle fatigue. The roles of fatigue crack initiation and growth are examined for materials that are nominally-defect-free, as well for materials that have experienced significant prior structural damage. An assessment is presented of the potential utility of the conventional threshold stress intensity factor range, ΔK _th, defined by testing specimens containing large cracks. Although the general utility of a large-crack-ΔK _th approach is questionable due to the potentially rapid growth of small fatigue cracks, the low allowable stresses involved in turbine engine high cycle fatigue appear to limit and simplify the small-crack problem. An examination is also presented of the potential effects of high-cycle fatigue and low-cycle fatigue (HCF/LCF) interactions.     

THE MECHANISMS AND DETECTION OF EMBRITTLEMENT IN Cr-Mo PRESSURE VESSEL STEELS

Abstract— 21/4CrlMo steel and 11/4Cr1/2Mo steel have been widely been used for hydro-processing units such as hydro-desulphurising and hydro-cracking reactors. These reactor pressure vessel steels have a potential for temper embrittlement that leads to toughness degradation and a reduction of the critical flaw size for brittle fracture. These steels are also susceptible to hydrogen embrittlement, especially in aged steels where cracks may propagate in the base metal up to the critical flaw size. A vessel with adequate toughness when originally constructed may therefore embrittle during service and such changes may require pressure restrictions during start-up and shut-down.
A survey of the literature shows composition to be the controlling parameter for both temper embrittlement (TE) and hydrogen embrittlement (HE), in-particular the presence of residual impurity elements such as P and the presence of elements such as Mo which nullify the effect of impurity segregation.
Much information is available to describe embrittlement phenomena for Cr-Mo steels. This paper reviews the mechanisms of TE and HE and describes a microstructural characterisation route which subsequently allows the structural integrity of potentially embrittled vessels to be examined for the purposes of remaining life assessment and plant life extension.