Metal Materials-6

1、6 Refractory Steels and Alloys在高温下长期工作不会因介质侵蚀而破坏的的钢叫热稳定钢，也叫耐热不起皮钢或抗氧化钢。在高温下仍具有足够强度不会大量变形或破断的钢叫热强钢16.1 Properties and Composition6.1.1 Working conditions High temperatures Mechanical loading Oxidation and corrosion Diffusion Phase transformation26.1.2 Properties requirement1. Heat resistance A. Stabi

2、lity at high temperature Oxidation resistance is the most important B. High temperature strength The capability to resist deformation and/or fracture at high temperature.3 a) Instantaneous strength 把试件装加热到预定的试验温度并保温，再进行拉伸所测得的强度指标，还可以同时测定塑性。方法：电炉加热（高温拉伸机） 通电加热（热模拟试验机）4b) Creep strengthThe ability to

3、resist deformation at high temperature.金属材料在一定的温度下受到一定应力作用时，随着时间的增长而缓慢地产生塑性变形的现象，称为蠕变。由于蠕变而导致的材料断裂，称为蠕变断裂。 Temperature above recrystallization temperature Stress greater than the yield strength at the temperature Time5Creep curve of metals 第一阶段，蠕变减速。加工硬化作用超过回复和再结晶。 第二阶段，恒速蠕变。加工硬化和回复再结晶相互平衡。 第三阶段，蠕变加

4、速。发生晶界滑动，出现空洞、裂纹，使承载能力下降，蠕变加速。微裂纹互相连接而形成晶间断裂，所以蠕变断裂常常表现为晶间断裂的特征。6Creep strength T temperature; Strain time T/通常采用在给定温度下和规定时间内使金属产生一定变形量的应力值作为金属在该条件下的蠕变强度。7c) Endurance strength The ability to resist fracture at high temperature for a long time. T temperature; timeT持久强度是指在给定温度下、经过规定的时间使金属发生断裂时的应力值。82

5、. Fatigue resistance at high temperature A. Mechanical fatigue Resulting from mechanical loading B. Thermal fatigue Resulting from the changing temperature T196.1.3 Alloying of Refractory Steels and Alloys 1. Improve the stability at high temperature Dense oxide film Cr, Al, Si Ceramic coating Al2O3

6、, ZrO2, TiO2 Increase the formation temperature of FeO (通常在570以上产生，Fe原子在其中扩散很快，加速氧化) Cr, Al, Si Add Cr and Ti into Ni-based alloys 10Rare-earth and Alkaline-earth metals: 提高耐热钢和耐热合金的抗氧化能力，特别在1000以上，可消除晶界优先氧化现象。W, Mo 降低钢和合金的抗氧化能力，WO3和Mo3O具有低熔点和高挥发性，使抗氧化能力变坏。11Effects of alloying elements on oxidation

7、 rate of steels12 400以上，水蒸气导致钢氧化： 3Fe+4H2O=Fe3O4+4H2 氧化产生的氢又会导致氢脆。抗氧化和气体腐蚀能力分级： 0.11.0mm/a 抗氧化 1.03.0mm/a 次抗氧化 3.010.0mm/a 弱抗氧化 10.0mm/a 不抗氧化 132. Improve high temperature strength KEY: Decrease diffusion rate a) Increase the bonding strength of matrix atoms b) Increase the stability of strengthenin

8、g phases A. Use the refractory elements as matrix Ni (1453oC), Cr (1890oC), Fe (1538oC), Co (1492oC), Nb (2468oC), Mo (2625oC), W (3410oC), Ti (1670oC), Zr (1852oC), Hf (2227oC), Re (3180oC)14B. Alloying and heat treatment a. Solid solution 奥氏体钢一般比铁素体钢具有更高的热强性，这是因为相原子排列较致密，原子间结合力较强的缘故。 溶质原子的熔点越高，溶质原

9、子与基体金属原子半径差别越大，溶质元素与基体金属的化学性质差别就越大，固溶强化效果越好。15b. Precipitation strengthening 细小、弥散、稳定的沉淀相具有较好的强化效果。 c. Boundary pinning 添加微量表面活性元素，使之富集于晶界。 e.g. ASK-doped Mo 定向结晶、使晶粒适当粗化。 d. Heat treatment Get the desirable grain size Improve the distribution, size, amount of the precipitation particles16AKS-doped

10、Molybdenum17 e. Strain hardening Warm working Solid solution, deforming (at the temperature lower than recrystallization temperature but higher than recovery temperature), stress-releasing temper High temperature ausforming Being worked in austenite state, then quenching and tempering to retain the

11、work-hardening effect.186.1.4 Categories 1. Steels Microstructure in use Pearlite & Ferrite + Pearlite steels （正火态使用） Martensite steels （淬火、回火后使用） Ferrite steels Austenite steels Application Oxidation-resistant steels Refractory steels Valve steels192. Alloys Matrix element Fe-based, Ni-based, Co-ba

12、sed Manufacturing process Wrought alloys Cast alloys 206.2 Refractory Steels6.2.1 Pearlite steels As-normalized: Ferrite + Pearlite Working at the temperature up to 600oC 0.10-0.40 C Alloying with Cr，Mo，W，V，Ti，Nb Strengthening ferrite Stabilizing carbides Preventing graphitization 需要高温强度和抗氧化性。211. T

13、ubes in boilers（锅炉管道） 1) Properties requirement High temperature strength and lasting ductility Oxidation and corrosion resistance Stable microstructure （防止石墨化？）（防止石墨化？） Formability (for hot working) 222) Composition (similar to high-strength, low-alloy steels) 0.2%C Alloying Solution strengthening

14、Precipitation strengthening Stabilizing ferrite Cr, Mo, W, V, Ti, Nb3) Heat treatment Normalization (Bainite) and temper at high temperature (precipitation hardening)232. Fastening parts（紧固件） 1) Properties Good yield strength: s2-2.50（预紧力） Anti-stress-relaxation Lasting ductility Low lasting notch s

15、ensitivity Oxidation resistance (anti-bite) 2) Composition Like quench and temper steels 35, 45, 35CrMoA, 25Cr2MoV 243. Rotors in gas turbine Uniform and isotropic mechanical properties High temperature strength and lasting ductility Stable microstructure Good quenchability Manufacturing properties

16、34CrMo, 33Cr3MoWV, 27Cr2Mo1V, 20Cr3MoWV 256.2.2 Martensite steels Good quenchability Martensite can result from air cooling Cr12 series steels Precipitation phases: M23C6, VC, NbC, nitrides High Mo and low W content results in high toughness and ductility, but low creep strength. Otherwise, high cre

17、ep strength, low toughness and ductility will result. Boron strengthens grain boundaries. 2Cr12MoV, lCr10Mo2VNb, lCr11W2VNbN, lCr9W2MoVNbNB 266.3 Oxidation resistance steelsGood oxidation resistance & Moderate strength at high temperatureWrought and/or cast steels 276.3.1 Ferrite steels Cannot stand

18、 impact very well Coarse grains Good corrosion resistance in oxidizing and sulfur-containing atmosphere e.g. 1Al3Mn2MoWTi 286.3.2 Fe-Al-Mn austenite steels 0.65-0.85C, 25-30%Mn, 6-10Al, 1.0-1.5W, 0.1Ti, 0.1RE Only a few carbide-forming elements Working temperature lower than 900oC: 6Mn28Al7TiRE (sin

19、gle austenite) Working temperature lower than 950oC: 6Mn28Al8TiRE (austenite and ferrite)Al: decreasing oxidizing rate and preventing carbonizing 296.3.3 Cr-Mn-C-N austenite steels Mn, C, and N substitute for Ni, so as to decrease the cost. e.g. 3Cr19Mn12Si2N, 2Cr20Mn9Ni2Si2N Si: decreasing oxidatio

20、n Working temperature up to 1000oC6.3.4 Cr-Ni austenite steels 3Cr18Ni25Si2, 1Cr25Ni20Si2, 4Cr22Ni4N, 3Cr24Ni7N Working temperature up to 1100-1200oC 306.4 Valve Steels6.4.1 Properties requirement inlet valve 300-400oC outlet valve 600-800oCHigh strength, toughness, hardness, and wear resistance at

21、working temperatureOxidation and corrosion resistanceStable microstructure and dimension under the changing temperatureCold and hot formability, weldability316.4.2 Martensite steels Cr and Si improve quenchability, corrosion resistance, temper resistance, and thermal fatigue resistance. Mo improves

22、high temperature strength and prevents temper embrittlement. e.g. 4Cr9Si2, 4Cr10Si2Mo, 5Cr8Si2, 5Cr9Si3 326.4.3 Austenite steels Austenite matrix and precipitation particles e.g. 4Cr14Ni14W2Mo, 5Cr21Mn9Ni4N, 2Cr21Ni12MnSiN Heat treatment: Solid solution Aging336.5 Superalloys Iron-based Nickel-based Cobalt-based Cast alloys and wrought alloys 高温合金通常具有fcc结构，所以铁基合金中要加入足够的Ni，钴基合金中也要加入适量的Ni，以得到稳定的奥氏体基体；又由于高温合金常常工作于腐蚀性环境中，所以需要加入一定量的Cr。因此，铁基合金实际上是Fe-Ni-Cr为基，镍基合金是Ni-Cr，钴基合金则是Co-Ni-Cr。 高温合金的主要强化手段有固溶强化、沉淀强化和晶界强韧化。346.5.1 高温合金的牌号 变形合金GH后第一个数字表示分类号 1-热稳定型铁