《金属材料学》第6章 耐热钢

,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,第,6,章 耐热钢及铁基耐热合金,蒸汽锅炉、蒸汽涡轮，航空工业的喷气发动机，,以及航天、舰船、石油和化工等工业部门的许多工,作部件在高温下工作。因为是在高温下服役，所以,这些零部件技术要求比较苛刻。通常把在高温条件,工作的钢称为耐热钢。,耐热钢应具有两方面的基本性能，一是有良好的,高温强度和塑性，二是有足够高的高温稳定性。根,据不同服役条件，常常将耐热钢分为热强钢和热稳,定性钢两大类。,6.1,基本概念,1,金属的抗氧化性,铁氧化物类型有,FeO,、,Fe,2,O,3,、,Fe,3,O,4,三种。,FeO,：结构疏松，冷却分解应力，结合力弱，易剥落；,Fe,2,O,3,、,Fe,3,O,4,：结构致密，和基体结合好。,570,以下，氧化膜由,Fe,2,O,3,和,Fe,3,O,4,组成；,570,以上，氧化膜由,Fe,2,O,3,、,Fe,3,O,4,和,FeO,氧化物组成，约,1:10:100.,570,以上铁的氧化过程大大地加速，遵守,抛物线规律,形,成,规,律,二、提高钢抗氧化性的途径,1,、,提高钢氧化膜稳定性,FeO,的形成温度,;, FeO,的形成温度,又优先形成稳定氧化物,Cr,、,Al,、,Si,。,途,径,例,子,如：,1.03,Cr,，,FeO,形成温度为,600,；,1.14,Si,，,FeO,形成温度为,750,；,Cr,、,Al,量高时，钢的表面可生成致密的,Cr,2,O,3,或,Al,2,O,3,保护膜。,2,、形成致密、稳定的氧化膜,Cr,、,Al,、,Si,、,Ti,等，逐步形成以合金元,素氧化物为主或合金元素氧化物的氧化膜，,如,Al,2,O,3,、,SiO,2,、,Cr,2,O,3,等致密、稳定。,Concepts and classifications,Heat-resistant steels and refractory alloys,:,Metallic materials possess required strength, oxidization-resistance and anti-corrosion ability at high-temp.,Category according to the quantity of alloy elements:,Low alloy pearlite-type heat-resistant steels:,Developed based on low alloy structural steels.,High alloy heat-resistant steels for special purpose:,Developed based on stainless steels.,Heat-resistant steels for special purpose:,Refractory steels(,热强钢,),and thermostable steels(,热稳定钢,)according to application properties.,Refractory steels:,在高温下,有一定抗氧化能力,并具有足够强度,而不产生大量变形或断裂的钢种，如高温螺栓、涡轮叶片等。,Thermostable steels:,在高温下,抗氧化或抗高温介质腐蚀而不破坏的钢种，如炉底、炉栅等。又称抗氧化钢。,Category according to crystalline structure:,Austenite-type,for oxidization resistance,Ferrite-type,Martensite-type,for high-strength,PH-type,(precipitation hardening),耐热钢和高温合金的使用温度范围十分宽广，从几百度到千度以上。一般来讲，耐热钢和高温合金工作温度是指该金属或合金的（,0.30.5,）,Tm,以上的温度。,Classifications of refractory alloys,According to type of basic alloy elements:,Fe-based refractory alloys: Basing on Fe, total amount of alloy elements is larger than 50,.,Ni(Co)-based refractory: Basing on nickel or cobalt.,According to processing methods:,Deformation refractory alloys,Cast refractory alloys,Powder-metallurgical refractory alloys,According to strengthening methods:,Solid solution strengthening type,PH-type,Intermetallics-type,Oxidization dispersion strengthing-type,Fiber strengthing-type,铁基、钴基和镍基合金的使用温度一般不超过,1000,，温度再高就必须选用难熔金属（指熔点高于,1650,的金属）或其合金了。,6.1.2,钢的热强性,一、钢的热强性能指标,1,、蠕变及蠕变极限,蠕变极限是试样在一定温度下和在规定的,持续时间内，产生的蠕变变形量（总的或残余,的）或蠕变速率等于某规定值时的最大应力。,例如：,表示在,700,时，持续时间为,10000 h,，产生蠕变总变形量为,1%,的蠕变极限。,2,、持久强度,持久强度极限是试样在一定温度和规定的持续时,间内引起断裂的最大应力值，,如：,=300MPa,，表示在,700,时，持续时间,为,100 h,的持久强度极限值为,300 MPa,。,3,、高温疲劳强度,在高温疲劳时只有条件疲劳极限，即把在某一规,定的循环次数（一般采用,10,7,10,8,次）下而不断裂时,的最大应力作为疲劳极限。,4,、应力松弛,应力松弛现象是在具有恒定总变形的零件中，,随时间延长而自行降低应力的现象。,蒸汽管道螺栓,需要经常拧紧,原子易发生扩散，,使弹性变形变成了,塑性变形,从而使,应力不断降低,二、提高热强性的途径,1,、强化基体,基体金属熔点,：,Tm,，金属原子间结合力。,铁、镍、钼基是依次,合金晶格类型,：,fcc-Fe bcc-Fe,奥氏体型钢要比,F,、,M,、,P,型钢的蠕变抗力高,固溶体结合力,：合金化固溶体原子间结合,力，,Mo,、,Cr,、,Mn,、,Si,作用较大,常用合金元素对工业纯铁蠕变极限的影响,2,、强化晶界,净化晶界：杂质易偏聚在晶界，形成易熔夹杂物,晶界强度。,RE,、,B,优先结合杂质，形成,高熔点化合物，异质晶核从晶界转入晶内,填充空位：晶界上空位多，原子易快速扩散。,B,易偏聚晶界，晶界空位。,3,、弥散强化,弥散质点，位错运动，强度 有效,性和程度的关键是质点的性质、大小、分布、,稳定性。,珠光体热强钢的蠕变强度,4,、热处理,6.1.3,耐热钢的合金化,Cr,：钢抗氧化性主要元素。形成致密而稳定的,Cr,2,O,3,。,T,，,Cr,。如,600650,，需要,5%Cr,；,800,时，为,12%Cr,。固溶强化,Mo,、,W,：低合金热强性重要元素。固溶强化，,析出稳定相。,Al,：抗氧化性的有效元素。,在,1000,时，,6%Al18%Cr,水平。冶金工,艺性，使钢变脆。 不能单独加入，作为辅助,合金化元素。,Si,：抗氧化性的辅助元素。但效果比,Al,还要有,效，使钢脆性大，用,Si,代替部分,Cr,。,G,化，,K,聚集长大。一般在,23%Si,。,用于渗碳罐，抗渗性好。,Ni,：获得奥氏体组织,Ti,、,Nb,、,V,：形成稳定,K,，钢松弛稳定性，,热强性,C,：较低温度时强化钢，,G,化，,K,聚集长,大 尽可能,C,。,6.2,热强钢,6.2.1,珠光体热强钢,按用途主要有锅炉钢管、紧固件和转子用,钢等几大类。,一、锅炉钢管用珠光体热强钢,高温和高压，高温烟气和水蒸汽,安全可靠, 抗氧化和耐蚀，组织稳定,工艺性能好,Strengthening methods:,Solid solution strengthening,Addition of W, Mo and Cr (less than 5%),W and Mo soluble in -matrix, increase atomic bonding strength, recrystallization temp. and creep resistance.,Cr,0.5%,significant strengthening effect,0.5%,weak,strengthening effect,26,(2),Precipitation hardening,MC,M,2,C,M,6,C,M,7,C,3,V, Ti, Nb,MC,W,Mo,M,2,C,and M,6,C(Fe,3,W,3,C and Fe,3,Mo,3,C),Lower content Cr,cementite (Fe,Cr),3,C,Higher content Cr (,2-3%),(Cr,Fe),7,C,3,.,27,(3) Microstructure hardening,12Cr1MoV,：,Aus. Temp. 980,Furnace,cooling,ferrite,+ pearlite;,Air cooling,granular bainite + a small amount of ferrite + martensite, sequent high-temp. tempering;,Quenching,martensite, sequent high-temp. tempering;,Creep-rupture strength:,M+HTT,GB+HTT,F+P;,Creep-rupture plasticity:,F+P,GB+HTT,M+HTT.,28,29,普遍,问题,组织不稳定，如片状,P,球化、,K,长,大、,G,化， 强度，管子爆裂,合金,化,0.2%C ,球化、,K,长大、,G,化,采用,Cr- Mo- V,系,Mo,、,W,、,Cr ,固溶强化，,K,长大和,G,化，,Cr,抗氧化性和耐蚀性；,V,Ti,直接作用,间接作用,形成稳定,MC,型,K,，,弥散强化，热强度,使更多,Mo,、,W,、,Cr,固溶，热强度,典型钢号,12Cr1MoVG,是用量较大的钢管,材料，,15CrMo,、,12CrMoV,等,二、紧固件用珠光体热强,紧,固,件,处于初紧预应,力状态工作,高的屈服强度、,松弛稳定性，,缺口敏感性小，,一定抗氧化性,C,Cr/ Mo/ V,常用钢：,35CrMoV,、,25Cr2MoV,钢常用作中压,气轮机的螺栓和螺母材料，,25Cr2Mo1V,钢,在高压电厂中广泛使用,三、转子用钢,转,子,主轴、叶轮,或整锻转子,过热蒸汽，复杂应,力（扭,/,弯,/,热等）,热强性，沿轴向、径向均匀一致,的综合力学性能淬透性。,常用钢：汽轮机叶轮常用中碳珠光体热强钢,制造。如,35Cr2MoV,、,33Cr3WMoV,钢,6.2.2,马氏体热强钢,一、叶片用钢,在,Crl3,型马氏体不锈钢的基础上进一步,合金化发展了,Crl2,型马氏体耐热钢。,15Cr12WMoV,、,2Cr12WMoNbVB,等，,工作温度可在,600,左右。,常用于大功率火力发电机组。,1. 1Cr13, 2Cr13,Good oxidization resistance and corrosion resistance, but bad microstructure stability,Manufacture turbine blades below 450,2.,Cr12-type,Based on Cr12 martensite heat-resistant steels,Addition of Mo, W, V, Nb, N and B,Manufacture turbine blades under 570,35,Effect of alloy elements:,W and Mo,Addition of W and Mo can eliminate Cr,7,C,3,and results in the formation of (C,Mo,W,Fe),23,C,6, precipitation hardening effect.,Part of W and W is soluble in M,23,C,6,and M,6,C and most of W and W are soluble in matrix(solution hardening).,36,(2),V and Nb,Formation of MC, precipitation hardening,(3),N,Formation of MN, precipitation hardening,(4),B,Grain boundary hardening,37,2Cr12MoV and 2Cr12WMoV,大型发电设备中，大口径厚壁高压锅炉蒸汽管道、汽轮机转子和蜗轮叶片，工作温度,500-580,。,1Cr9W2MoVNbNB,多元复合合金化，具有高的蠕变强度，高压锅炉用钢。,38,二、气阀钢,内燃机排气阀工作条件非常苛刻,:,700850,工作，受到燃气的高温腐蚀、,氧化腐蚀和冲刷腐蚀磨损,热疲劳 。,常用钢号有,4Cr9Si2,、,4Cr10Si2Mo,，采,用,1050,淬火加热、油冷，,700,左右回,火，组织为回火索氏体，保持马氏体形貌,特征。高于,750,的阀门需要,采用奥氏体型,热强钢。,6.3,抗氧化钢,抗氧化钢用于工业炉的构件、炉底板、炉罐等,.,F,型是在,F,型不锈钢,基础上发展形成。,A,型抗氧化钢是在,A,不锈钢基础上发展起来的。,A,型抗氧化钢目前主要有铬,-,镍系和铬,-,锰,-,氮系。铬,-,锰,-,氮系奥氏体型抗氧化钢在,950,以下有较,好的抗氧化性，且有较高的高温强度，工艺性不如,铬,-,镍系抗氧化钢。低镍奥氏体抗氧化钢，使用比较,多的有,ZG4Cr22Ni4N,和,ZG3Cr24Ni7N,钢，经济。,6.4 PH heat-resistant steels 6.4.1 Carbide PH heat-resistant steels,Typical steels,4Cr13Ni13Co10Mo2W3Nb3,4Cr13Mn8Ni8MoV3Nb(GH36) (Ni replaced by Mn),Strengthening mechanism,(Nb,V)C,precipitating at 670-700,another carbide M,23,C,6,41,42,6.4.2 Intermetallics PH heat-resistant steels,Chemical composition,Austenite containing Al and Ti,43,Strengthening mechanism,Coherent precipitation of -Ni,3,(Ti,Al) during aging process.,The lattice constant of -Ni,3,(Ti,Al) is closer to the matrix.,44,45,46,47,48,6.5.1 Background,The maximum temp. of Fe-based heat-resistant alloys is 750-850,Ni-based alloys were developed for higher service temp.,Basing on Cr20Ni80 alloy,Addition of a large amount of elements in order to strengthen the alloy, such as W, Mo, Ti, Al, Nb, Co, Ta.,49,6.5 New type heat-resistant alloys,6.5.2 Effect of alloy elements,Precipitation phase -Ni,3,(Al,Ti),Al and Ti,-Ni,3,(Al,Ti),Coherent precipitation,Low interfacial energy,Low growth rate at high temp.,Good plasticity,-Ni,3,Ti,Al/Ti,1,50,51,2.,Solid solution strengthening,Addition of W, Mo and Cr improve atomic binding force, reduce atomic diffusion rate, and form solid solution strengthening effect.,52,3.,Function of Co,Form -(Ni,Co),3,(Al,Ti), more stable.,Co increases the amount of .,Co decreases fault energy, and increases fault width.,4. Function of Cr,Improve oxidization resistance,14-20wt%Cr, 500-700, form dense Cr,2,O,3,800-1000, inner layer Cr,2,O,3,outer layer NiO Cr,2,O,3,53,5. Impurity elements or low melting metals,Pb, Sb, Sn, Bi strongly reduce strength of grain boundary, high-temp impact toughness and high-temp. plasticity.,Aggregation at grain boundary reduces diffusion activation energy of atoms at grain boundary, and reduces creep rupture strength.,54,55,6.5.3 Typical alloys and heat treatment,56,57,本章小结,温度对钢的使用来说有很大的影响，是关键因,素。在高温条件下工作的零部件就其材料成分、,性能要求及失效机理和一般常温下的零件相比，,有很大的不同。,钢的高温强度和组织稳定性是耐热钢设计与处,理工艺的主要矛盾。从合金化考虑，主要措施有,原子结合力、强化晶界、稳定相的弥散强化。,不同合金的耐热钢有其适宜的使用温度范围。,