Metal Materials-6

1、6 Refractory Steels and Alloys在高溫下長期工作不會(huì)因介質(zhì)侵蝕而破壞的的鋼叫熱穩(wěn)定鋼，也叫耐熱不起皮鋼或抗氧化鋼。在高溫下仍具有足夠強(qiáng)度不會(huì)大量變形或破斷的鋼叫熱強(qiáng)鋼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 把試件裝加熱到預(yù)定的試驗(yàn)溫度并保溫，再進(jìn)行拉伸所測得的強(qiáng)度指標(biāo)，還可以同時(shí)測定塑性。方法：電爐加熱（高溫拉伸機(jī)） 通電加熱（熱模擬試驗(yàn)機(jī)）4b) Creep strengthThe ability to

3、resist deformation at high temperature.金屬材料在一定的溫度下受到一定應(yīng)力作用時(shí)，隨著時(shí)間的增長而緩慢地產(chǎn)生塑性變形的現(xiàn)象，稱為蠕變。由于蠕變而導(dǎo)致的材料斷裂，稱為蠕變斷裂。 Temperature above recrystallization temperature Stress greater than the yield strength at the temperature Time5Creep curve of metals 第一階段，蠕變減速。加工硬化作用超過回復(fù)和再結(jié)晶。 第二階段，恒速蠕變。加工硬化和回復(fù)再結(jié)晶相互平衡。 第三階段，蠕變加

4、速。發(fā)生晶界滑動(dòng)，出現(xiàn)空洞、裂紋，使承載能力下降，蠕變加速。微裂紋互相連接而形成晶間斷裂，所以蠕變斷裂常常表現(xiàn)為晶間斷裂的特征。6Creep strength T temperature; Strain time T/通常采用在給定溫度下和規(guī)定時(shí)間內(nèi)使金屬產(chǎn)生一定變形量的應(yīng)力值作為金屬在該條件下的蠕變強(qiáng)度。7c) Endurance strength The ability to resist fracture at high temperature for a long time. T temperature; timeT持久強(qiáng)度是指在給定溫度下、經(jīng)過規(guī)定的時(shí)間使金屬發(fā)生斷裂時(shí)的應(yīng)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以上產(chǎn)生，F(xiàn)e原子在其中擴(kuò)散很快，加速氧化) Cr, Al, Si Add Cr and Ti into Ni-based alloys 10Rare-earth and Alkaline-earth metals: 提高耐熱鋼和耐熱合金的抗氧化能力，特別在1000以上，可消除晶界優(yōu)先氧化現(xiàn)象。W, Mo 降低鋼和合金的抗氧化能力，WO3和Mo3O具有低熔點(diǎn)和高揮發(fā)性，使抗氧化能力變壞。11Effects of alloying elements on oxidation

7、 rate of steels12 400以上，水蒸氣導(dǎo)致鋼氧化： 3Fe+4H2O=Fe3O4+4H2 氧化產(chǎn)生的氫又會(huì)導(dǎo)致氫脆。抗氧化和氣體腐蝕能力分級： 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 奧氏體鋼一般比鐵素體鋼具有更高的熱強(qiáng)性，這是因?yàn)橄嘣优帕休^致密，原子間結(jié)合力較強(qiáng)的緣故。 溶質(zhì)原子的熔點(diǎn)越高，溶質(zhì)原

9、子與基體金屬原子半徑差別越大，溶質(zhì)元素與基體金屬的化學(xué)性質(zhì)差別就越大，固溶強(qiáng)化效果越好。15b. Precipitation strengthening 細(xì)小、彌散、穩(wěn)定的沉淀相具有較好的強(qiáng)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 定向結(jié)晶、使晶粒適當(dāng)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 （正火態(tài)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 需要高溫強(qiáng)度和抗氧化性。211. T

13、ubes in boilers（鍋爐管道） 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（緊固件） 1) Properties Good yield strength: s2-2.50（預(yù)緊力） 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 高溫合金通常具有fcc結(jié)構(gòu)，所以鐵基合金中要加入足夠的Ni，鈷基合金中也要加入適量的Ni，以得到穩(wěn)定的奧氏體基體；又由于高溫合金常常工作于腐蝕性環(huán)境中，所以需要加入一定量的Cr。因此，鐵基合金實(shí)際上是Fe-Ni-Cr為基，鎳基合金是Ni-Cr，鈷基合金則是Co-Ni-Cr。 高溫合金的主要強(qiáng)化手段有固溶強(qiáng)化、沉淀強(qiáng)化和晶界強(qiáng)韌化。346.5.1 高溫合金的牌號 變形合金GH后第一個(gè)數(shù)字表示分類號 1-熱穩(wěn)定型鐵