薄钢板直接电加热熔覆工艺研究

收稿日期:　20030728作者简介:　陈建华(1976),男,工学硕士,现任南京工程学院电力系讲师,主要从事电气控制方面工作。

薄钢板直接电加热熔覆工艺研究陈建华,赵冰华(南京工程学院,江苏南京　211100)[摘　要]　在自制的电加热设备上对薄钢板的熔覆工艺进行了初步试验,并对试验材料进行了组织分析和性能分析,试验结果证明了这种熔覆工艺的可行性。

[关键词]　薄钢板;直接电加热;熔覆;表面处理[中图分类号]TG156.9 [文献标识码]A [文章编号]10023364(2003)12004802 直接电加热热处理是利用电流在工件中通过时,由于工件的电阻而产生热量使其加热,以达到处理工件的目的。

这种加热方法的优点在于加热速度快,加热至高温所形成的奥氏体晶粒很细,经热处理后钢的韧性可提高1.5～2倍。

此外,由于工件在加热装置上所需的时间很短,因加热而产生氧化、脱碳的可能性减少,有利于提高工件的表面质量[1]。

直接电加热的热处理工艺已应用多年,对其在化学热处理中的应用,国内外均进行过试验研究。

但以往的研究大多是针对毛坯或大型钢件,很少应用于较精确的工件。

本文通过自行研制的电热设备,在薄钢板表面上熔覆易熔合金或铝合金,以使钢材获得优良的耐磨及抗高温氧化和腐蚀能力,同时对其工艺实施中的一些特点以及所获熔覆层的性能进行了分析,并将其应用于实践中。

1　试验方法电加热所用的熔覆材料为Ni60A 易熔合金和WC 混合粉末,前者的化学组成(Wt %)为:C 0.46%、Cr 15%～18%、B 3%～4.5%、Si 0.24%、Fe 14.3%、其余为Ni ;后者为钴包碳化钨粉,其中Co 占4%。

试验采用的混合粉末中WC 的比例有12.5%、25%、50%3种,电加热用的基体材料为45号钢,电加热输出电压为15V ,加热时间为5s ,试样面积为200mm ×10mm ,厚度3mm ,试验前将用有机溶剂调制好的熔敷涂料均匀涂敷在试样上。

直接电热设备市面上尚无专门的型号供购买,自制的电热设备输出电压为(2～36)V ,电流输出最大值为160A 。

图1为自行研制的直接电加热装置[4]。

图1　直接电加热设备电加热后试样的硬度测定在HR 150A 洛氏硬度计上进行,初试验力98N ,总试验力147N ,保载10s ;在ML S 23型湿砂橡胶轮磨损实验机上测定耐磨料磨损性能,采用(425～212)μm 石英砂,砂浆比为1.5∶1.0(单位kg );橡胶轮直径为177mm ,厚度为12.82mm ,硬度为HS60,主轴转速240r/min ,接触正压力68.6N ,预磨1000r/min ,正式磨2000r/min 。

试样的耐磨损能力按其失重及相对耐磨性(以纯镍基易熔合金为基准)进行比较。

冲蚀试验所用试样面积为25mm ×10mm ,厚度3mm ,表面用金相砂纸磨光。

试验条件如下:石英砂粒平均直径750μm 、气流速度50m/s 、喷射角30°、试样与喷嘴之间的距离为9mm 、冲蚀时间(20～30)min ;对于冲击每块试样的总砂量控制在(160～180)g 。

进技术交流　ο}　热力发电・2003(12)行冲蚀后,用丙酮对试样进行清洗,待干燥后用精度万分之一的天平称重,失重量按多块试样失重的平均值确定并由此确定冲蚀率。

作为对比,对Cr12MoV 钢试样也进行了常温冲蚀试验。

2　试验结果2.1　组织分析熔烧后的试样表面一般来说不平整,四周高,中部略低,进行组织和性能分析前应先将试样表面磨平,然后制备金相试样。

金相组织观察表明:覆层的组织中Ni 基合金主要以枝晶的形式存在,其间夹杂着未熔化的WC 颗粒。

图2a 为含12.5%WC 试样覆层的组织,白色部位为枝干,黑色部位为Ni 基合金加WC 颗粒。

可以看出,距离基体(全白色)越近,枝晶变得越粗。

这可能是由于覆层内表面凝固时过冷度较小,形核部位较少的原因。

此外,能谱分析结果表明,覆层组织中Fe 原子的百分含量有所增加,在靠近基体的局部部位可高达74.2%,覆层与基体之间出现成分稀释的结果会导致合金的熔点降低,使枝晶得以充分长大。

一般来说,1250℃下熔烧的试样,其组织中的枝晶要比1200℃下熔烧的粗大。

同样,熔烧时保温时间延长到10min ,也存在着枝晶长大的倾向。

图2b 为含50%WC 试样的组织。

该试样为熔烧后试样凸起部分被磨掉后的表面,显示了覆层从表面向心部组织的变化。

图中黑色部位是表层脱落形成的,表明当WC 含量较高(50%)时,覆层组织比较疏松。

黑色部位与基体之间的覆层组织特征符合一般凝固规律,靠近基体处枝晶较为粗大。

图2　试样金相组织覆层内的显微硬度与碳化钨分布有关。

组织中大块的颗粒为HV730左右;小而密集的颗粒处,硬度在HV600～640之间;纯Ni 基的显微硬度为HV270,而其内混有网状枝晶时,硬度提高HV15～20;与覆层相连的基体平均硬度为HV416。

2.2　性能分析对WC 含量不同的试样进行了硬度、耐磨性和抗冲蚀性能试验,其结果分别见表1～表3。

表1　试样硬度比较熔覆涂层成分12.5%WC 25%WC50%WC 纯Ni 基合金HRC 38405528表2　耐磨性比较熔覆涂层成分12.5%WC 25%WC 50%WC纯Ni 基合金相对耐磨性ε0.7120.9表3　抗冲蚀性比较熔覆涂层成分12.5%WC 25%WC 50%WC 纯Ni 基合金Cr12MoV 冲蚀率/g ・g -10.8×10-52.5×10-54×10-51.5×10-5 1.8×10-5从表可见:WC 含量增加对试样表面的洛氏硬度值影响较大,在同等条件下,50%WC 试样的硬度最高,可达HRC55,纯Ni 基试样的硬度最低,仅为HRC28;随WC 含量的增加,覆层的耐磨性增加,含50%WC 的试样耐磨性最高,可达纯Ni 基合金的2倍之多;含50%WC 的试样抗冲蚀性反而较差,也就是说不能单纯从硬度来判别材料的抗冲蚀能力,特别是对于含硬质相的复合材料,由于冲蚀过程包括了金属变形过程的行为,如挤压、锻薄、弯曲和断裂的复杂过程,而不完全是单纯的微切削[5]。

当材料中的硬质相与基体结合不太牢固时,其抗冲蚀能力必然下降。

3　结　语针对薄钢板的形状特点采用特殊的加热方法对其进行表面强化,这是常规的表面强化手段不可能达到的。

试验结果表明,这种直接电加热设备基本上能完成薄钢板的熔覆工艺,并可获得较为满意的结果。

[参　考　文　献][1]　东北工学院金属学教研室.热处理车间设备与设计[M ].中国工业出版社,1995.[2]　KN INH.NH.金属与合金的电化学热处理[M ].北京:机械工业出版社,1987.[3]　魏兴钊,蒙继龙.钢的直接电加热快速渗铝的研究[J ].金属热处理,1995,(7).[4]　X.M.约非.金属的加热(俄文)(苏)[M ].冶金出版社,1981.[5]　A.V.Levy.Erosion and Erosion Corrosion of Metals[J ].Corrosion ,1995,(11):872892.技术交流热力发电・2003(12)ο∼　surface area and air -leakage ratio in furnace as examples ,the influencing factors upon temperature of flue gas from utility boilers have been analysed and cal 2culated.DEVE LOPMENT AN D APPL ICATION OF H YDRAU L IC TENSI LE B OLTS FOR SHAFT COUPL ING S OF TURB O -GENERAT OR SETS XU Wei et al (43)………………………………………………………………………………………………………………………The problem of difficult dismount for shaft coupling ’s bolts of high -capacity turbo -generators is always a puzzling matter in a comparatively large range of installation and repair works at power plants.Through efforts of engineers and technicians with Dongfang Steam Turbine Works and Shangan Power Plant ,a new kind of hydraulic tensile bolts has been developed to sabstituee the traditionally used couplings bolts.The new bolts have been successfully used on 300MW units in Shangan Power Plant.MECHANISM ANA LYSIS OF PITTING ATTACK DAMAGE FOR B LADES OF 300MW STEAM TURBINE AN D PREVENTIVE MEA 2SURES THERE OF CHENG Shao -bing et al (46)…………………………………………………………………………………………Through approaching the mechnism of pitting attack damage for blades of a 300MW steam turbine in a coastal power plant ,it is pointed out that the main cause leading to pitting attack damage is the result of chorine ions attack ,and some preventive measures ,induding strengthening feed -water quality supervi 2sion ,enhancing the steam quality ,and strictly controlling chlorine ions content in the steam etc.,have been given.STU DY ON TECHN OLOG Y FOR ME LTING AN D COVERING THEN STEE L PLATE B Y DIRECT LY E LECTR O -HEATING CHEN Jian -hu a et al (48)………………………………………………………………………………………………………………Preliminary test for the title technology has been carried out with the self -made electro -heating facility.The structure and behavior of the tested material has been analysed.The results of study have proved the feasibility of said technology.CAUSE ANA LYSIS OF WE LDE D SEAM CRACKING FOR COMB -SHAPE D SEA L PASSING THR OUGH THE R OOF OF RE 2HEATER L IN Ding et al (50)………………………………………………………………………………………………………………Directing against the problem of leakage due to cracking frequently occured on welded seams between tubes of medium -or high -temperature reheater and comb -shaped sealing plates in D G 1025/18.2-Ⅱ(3)type boiler ,macro -analysis and micro -analysis have been carried out for the sealing structure and the crack fracture.It is considered that the cause of failure to be creep rupture under the effect of structural geometric stress ,and the type of rupture to be wedge -shaped creep rupture.For this reason ,a mode of changing the welded structure between the comb -shaped sealing plates and the tubes can be used to eliminate or reduce the structural geometric 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layer ;(4)under the test condition ,the desulphuation efficiency of activated carbon reaches 99%;(5)the activated carbon soaked in HNO 3with suitable concentration can enhance the ability of activated carbon to absorb SO 2.COMPARISON STU DY ON THE THERMA L STABI L IT Y OF CHINA MADE ZR -881T YPE FIRE RESISTANT OI L WITH THAT OF TWO KIN DS OF FIRE -RESISTANT OI L IMPORTE D FR OM ABR OAD YANG Dao -w u et al (57)……………………………The acid value of three kinds of fire -resistant oil ,i.e.ZR -881type fire -resistant oil made in China.as well as that made by A KZO Chemical Co in U.S.and that by FMC Co in U.K.,has been measured under different temperature and different time.There are two stages ,namely induction stage and speed up stage ,in accelerated oxidation reaction.In the induction stage ,the acid -forming reaction belongs to 0grade reaction.The comparison study shows that the thermal stability of ZR -881type fire -resistant oil is better than that of others.TEST STU DY ON USING WATER FR OM THE GRAN D CANA L AS ING OING WATER OF THE REVERSE -OSMOSIS SYSTEMXU Jian -xue et al (60)…………………………………………………………………………………………………………………The pretreatment scheme of using water from the Grand Canal as ingoing water of the reverse osmosis system has been studied.The main points are elimina 2tion of organics in the water ,and reduction of the water turbidity ,including :(1)determining the content of organics in raw water and the distribution range of its molecular weight ;(2)selecting cogulant and its concentration ;(3)defining the effects of cogulating aid and low turbidity additives ;(4)considering the infuence of p H value in raw water upon the cogulating effectiveness ;(5)conducting oxidation test for NaCIO ;(6)performing test of organics abosorbing abil 2ity for activated carbon.Based on the results of test ,an effective and practical pretreatment process has been put forward.STABI L IZATION TREATMENT OF HEAV Y META LS IN F LY ASH FR OM MSW INCINERATION L I Jian -xin et al (64)………Taking the stabilization treatment of heavy metals in fly ash from municipal solid waste (MSW )incineration as an objective ,various methods of fly -ash treatment on the world at present have been analysed ,providing necessary theoretical basis for non -injurious treatment of fly ash from MSW incineration ,and for further utilizing the said fly ash as a resource.CAUSE ANA LYSIS OF MA LFUNCTION CONCERNING R OT ORS SING LE POINT GR OUN DING PR OTECTION FOR GENERAT ORS L IU Shi -xiong et al (69)……………………………………………………………………………………………………………………The causes leading to malfunction concerning rotor ’s single point grounding protection for four generators in the second -phase of Shenhua Dongdaliuta Ther 2mal Power Plant have been analysed.It is considered that the main cause is the shaft of generators hasnt actually earthed.The leading wires of carbon brushes installed on the shaft and making contact with the shaft have been connected into the protectiion device after earthing firstly through resistance 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complex vibration concerning induced draft fan of 300MW units in Shimen Power Plant ,the causes leading to said vibration have been found through vast amount of test and analysis ,and some remedial measures ,such as strengthening the stiffness of supports ,adjusting the centre of shaft ect ,have been put forward After retrofitting ,the influence of excessive vibration concerning induced draft fan upon units operation hasn ’t occurred a 2gain ,and the economic benefits being remarkable.SOME PR OB LEMS NEE DING T O PAY ATTENTION IN THE WORK OF FUE L ANA LYSIS ZHU Xiu -ming (78)…………………The source of erros appeared in fuel analysis has been analysed.Directing aganst the problems existing hi links such as sample collection ,preparation ,and labo 2ratory test etc ,,some recommendations have been put forward as follows :(1)strengthening propaganda of the new standard ;(2)allocating mechanical e 2quipments for sample collection and preparation ;(3)chemical personnel should realize sample 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