Repair Effect of Hot Work Tool Steel by Laser-Melting Process

J. Mater. Sci. Technol, Vol.19 Suppl1, 20039Repair Efect of Hot Work Tool Steel by Laser-Melting Process低| AYahong SUN)t, Satoshi HANAKP), Hitoshi UCHIDA), Hisakichi SUNADA1) and Nobuhiro TSUJI2)1) Graduate School of Engineeing, Himeji Institute of Technology 2167, Shosha, Himeji 67-2201, Japan2) Technical Research Laboratories, Sanyo Special Steel Co, LTD. 3007, Nakajima, Shikama-ku, Himeji 672-8677, Japan[Manuscript received August 20, 2003)Both wear and crack due to heat checking in hot work tool steel are major failure modes. It is desirable to find amethod to lengthen the tool life while reducing manufacturing cost. This paper suggests a method to improve toollife for hot work tool steel (SKD6) with crack by laser-melting process. The method has been evaluated using theimpact and fatigue test results. It is demonstrated that a repair of the crack by a laser. melting process is effectivefor life extension of the damaged tool.KEY WORDS: Hot work tool steel, Laser melting process, Impact test, Fatigue test, Fractography1. Introductionh 2400Die-casting, which has high size accuracy as compared700with otherCast, is economically in mass manufacture ofI 2200automobile parts.Since metallic molds used for die-casting600王王-T2000are used at high temperature repeatedly, crack damage on thesurface of molds, so-called heat checking has been a problem主in service.500t 1800The repair by arc welding has been commonly performedt 1600400for tool, machine elements and so forth. However, this methodt 1400has a defect which may change its metallographic structureas a result of heating over a wide range. .A methodi of locally300+ : Hardnesst 1200melting selected area using laser beam, in addition the workI✧: Tensile strengthcan be possible in air, has attracted attention for repair to-00L400500000000day. Henze et al.I1] showed that an arc welding technique torepair forging, die using Robot guarantees a near net deposit.Tempered lemperature 1KErnst et al.2 studied a laser repair process on die surfaceFig.1 Tensile strength and hardness plottcd as a functionwith notch. Brown et al.(3] has described that the laser weldof tempering temperaturerepair of fatigue cracks in ship steel could reduce the cost andthe repair time. The authors(4.5 have demonstrated that thelaser processing is a possible way to repair faws like fatiguecracks in hot work tool steel. In this report, to find a pos-sibility of crack :pair of hot work tool steel using Nd:YAGlaser beam melting, the mechanical, impact and fatigue testswere carried out fractographically.2. Experimental500u1Commercial hot work tool steel SKD6 in Japanese Indus-trial Standard was used as test material. The chemical com-position was C: 0.37, Si: 0.94, Mn: 0.42, P: 0.013, S: 0.001,Ni: 0.14, Cr: 4.95, Mo: 1.15, V: 0.5 and Fe: balance in mass10umfraction. The material was heated at 1303 K for 3.6 ks andthen two stepwise tempering, 873 K and 828 K for 1.8 ks,The V-notched impact test specimens (5 mmx6 mmx55 mm) with fatigue pre crack were machined from CT speci-men. The fatigue test specimens with U-notch, that is circum-ferential groove in 2 mm depth and 1 mm radius, were alsomachined. Laser processing was done using Nd:YAG laserFig.2 Optical micrographs of laser-processed hot work toolwhich was focused on the surface, with the output power of0.5 kW and moving speed of 10 mm/s. In order to investigatesteel (a) macroscopic view, (b) melted zone I, (c) HAZthe impact characteristics after laser processing, the impactII, (d) HAZ II, and (e) based metal IVtest was performed at 300 K and 873 K using test machinein 5 kgf-m capacity. The fatigue test was performned to inves-tigate the change of fatigue properties after laser processing.3.FUsing a cantilever rotating fatigue machine, the test in air中国煤化工was performed under constant stress at 175 r/min.3.1 SFuguos OUwS a uag u u vensile“0HCNMHGle strength and thet Ph.D., to whom correspondence should be addresed,hardness 88 a function of tempering temperature. It can beE-mail: son@mech.eng.himeji-tech.ac.jp.92J. Mater. Sci. Technol, Vol.19 Suppl.1, 2003Table 1 Summary of Charpy impact energy1000TestCharpytemperature impact energy/K/(J/cm2)乏800V-notch30034873128V-notch with2日600pre-crack50Laser repairHAZ| Melted zone HAZ1174004-之873 K temperedDistance from center/mmafter repair103Quenched and62Fig.3 Hardness distribution of laser- welded zonetempered after repair97Table2 Ratio of crack initiation energy U1 andr: V-notcpropagation energy U2 to absorbed energy120f. ◆V.notch wih perack,Uo0o-80-Specimentemperature U:/Uo U2/Uo50-/(%)__ (%)404. 2042584(60200 400600800 100017831000Test temperature1K3763818Fig.4 Charpy impact strength plotted as a function of test2674temperature87_58seen that the hardness exhibits a maximum value at 773 K,and then these values fall abruptly to 873 K. From this result, the secondary hardening temperature of test materialessary.. Although the impact value with pre-crack did notis predicted to be about 773 K. The tendency of change ofchange in 300 K when the laser-processing was carried outboth tensile strength and hardness relative to the temperingbeforehand, the impact value was recovered in 873 K. In ordertemperature is well in agreement.to recover the impact strength at 300 K, two kinds of speci-3.2 Structuremens were prepared: (1) 873 K heated after laser processingFigure 2(a) shows the macroscopic structure of laser-and (theand (2) quenched and tempered after laser processing as thprocessed hot work tool steel. The melted zone depth is largersame condition as non-processed specimenas compared with the bead width, and it is suggested thata result, both treatments were able to recover their impactthe localized melting by laser beam is possible to annihilatestrength to the same extent.The impact fracture is often divided into the process ofthe defect near the surface. The welded zone is roughly di-crack initiation and propagation. It was measured for suchvided into three zones, i.e., (1) melted zone, (2)(3)HAZ (heataffected zone) and (4) base metal zone. The melted zonefracture process from an instrumented Charpy impact test,of Fig.2(b) reveals as dendrite pattern including martensiticthat is load-displacement curve. Up to the highest load point,structure microscopically that was heated above Ag line ande and it is considered as crack initiation process and as crack prop-then cooled.. Fig.2(c) and (d) reveal HAZ, where the grainsagation process after that. Table 2 shows the ratio of crackgrow bigger, rough and transformed to martensite structureinitiation and propagation energy to total absorbed energy,in the near melted zone side. On the other hand, in the nearU/Ua and U2/Uo, resU2/Uo, rspetively.y. In V-notched specimen, thebase metal zone side, it is tempered martensite pattern，as is .totalabsorbenergy is consumed for crack initiation at300 K, and for crack propagation at 873 K. In pre-crackedshown in Fig.2(e).Figure 3 shows hardness distribution with a position ofspecimen, however, the ratio of absorbed energy is low at both0.2 mm beneath the surface of laser-welded zone in Fig.2(a).temperatures. Most of absorbed energy will be consumed forThe melting zone at the center is the abruptly heated andthe crack propagation rather than the crack initiation. As forcooled structure, which is hardened more remarkably thanthe specimen repaired by laser processing, the absorbed en-the base metal zone. The hardness decreases sharplyin the ergies at 300 K are exhausted for crack initiation, but thoseat 873 K are improved. The ratio of the absorbed energy forzone from the melting zone to the HAZ.heat-treated specimen after laser processing is recovered tonearly the same value as that of V-notched specimens. From3.3 Impact strengththese results, it is recognized that the specimen repaired byFigure 4 shows the change of the Charpy impact strengthlaser8s a function of testing temperature. The number in thelaser processing seems to behave as the process of impact frac-parenthesis indicates pre crack length. The impact strengthture similar to the V-notched one.also increases with the rising of test temperature. The com-parison of the absorbed energy at 300 K and 873 K is shown.4 I中国煤化工in Table 1. The absorbed energy of pre- cracked specimen fallssing three kinds of specremarkably as compared with that without one at both tem-MHC N M H G heated condition afterperatures. This is because the degree of stress concentration laser proong uauuitiu w U8oe metal. The S-N curvesto induce crack initiation is higher than V-notched specimen,obtained are shown in Fig.5. The solid line in the Fig.5 is .and because the energy for initiating crack becomes unnec- determined by statistical methodl. It is confrmed that theJ. Mater. Sci. Technol, Vol.19 Suppl.1, 20039:of heat- checking, which is main cause of the failure of dies.Therefore, we can conclude that the laser processing method7is available for the extension of die life.4. Conclusions生suo(1) The laser-welded zone serves quenched structure andhe secondary hardening ocurre at773 K(2) Although the impact strength of the specimen withfatigue pre-crack become remarkably low, the heat treatmentafter laser-repairing can be made to recover impact strength.| + Laser-pocess(3) When the laserprocessed specimen was heat-treatedo: 773K tempcrest secondary hardening temperature, the fatigue strengthcould be recovered successfully.Number of cycles 1o talureFig.5 S-N plots of hot work tool steel under various condi-AcknowledgementtionsThe authors would like to acknowledge professor S.Katayama,Joining and Welding Research Institute, Osaka University, for laser-fatigue strength of laser processed specimen at 107 cycles de-processing and helpful discussion.ot non-processedone. A rise of the notch sensitivity of quenched structure pro-REERENCESduced by laser processing can be considered in addition tothe infuence of tensile residual stress on the surface layerl4. [1] A.Henze, H.Haferkamp, M.Niemeyer and W.Bach: Proc. 5thNext, when specimen was heat-treated at secondary harden-Int. Conf. on Tooling, 1999 ,453.ing temperature after laser processing, the fatigue strength[2] G.Ernst, A.Luftenegger and R.Ebner: Proc. 5th Int. Conf. oncould be recovered sssfully. This could be due to a high1 PMBrowyield strength resulted from a secondary hardening effect.] P.M.Brow, G.Shannon, W.Deans and J.Bird: Welding in theThe fractographic results also demonstrate that the far1! Y.Sun, H.Sunada and N.Tsuji: ISI Int, 2001, 41, 1006.tigue strength of hot work tool steel can be recovered by heati Y .Sun. S.Hanaki, HXsun,S Hanaki, H.Uchida, H.Sunada and NTsuji: ISL Int,treatment after laser processing". These procedures can becarried out at relatively low cost and effective for the repair[6 ] S.Nishijima: J. JSME, 1984, 50A, 1303.中国煤化工MYHCNMHG