Laser surface alloying of a low alloy steel with cobalt

ELSEVIER

.lournal of Maleri;ds Processing Tedmology 58 ~996) ~3U Q35

N ateriNs P ess g Tech=elegy

Technical Note

Laser surface alloying of a low alloy steel with cobalt Ding Peidao, Liu Jianglong, Shi Gongqi, Zhou Shouze, Cao Pengjun Del,artmem t!f MetallutXr am/Materi, els Engineering, Chongqing Universit),, Chongqing. Sic~man 630044, Peoph,'s Republic Ot China

lndustria| summary Features of laser surface alloying on a 0.21C-1.46Cr-3.52Ni-0.80W steel with cobalt powder are studied. Experimental resulls show that laser surface cobaltizing is possible and that the cobalt content of the surlace alloy may achieve 3.10 wt.%-5.63 wt.%. The maximum cobalt content attained is 9.96 wt.%. After alloying with cobalt, the performance of the surtace at elevated temperature is clearly improved. The hardness at 700 °C is 880 MPa, while the latigue performance in the p~escncc of thermal cycles is improved by 160%, when compared with the surface of the untreated steel. Keywords: Laser surface alloying; Cobalt; Performance; Microstructure I. Introduction

Laser alloying of substrates of metals with a variety of elements, such as carbon [1], nitrogen [2], silicon [3], and chromium [4], and of alloying compounds, tbr example W - C o ~Cr~-V [5], Ni-Cr~-B Si [6], and N i Cr.~AI~- F'e [71, has been demonstrated as all effective method of modification of tile surface properties. Tile Ilardness, wear resistance, and corrosion resistance of materials can be signi[icantly improved. Laser surlace alloying has been studied by many workers, including the behaviour of the melt [8,9], and the characteristics of the microstructure and performance of the surface at room temperature [10,1 1]. The technique of laser suri'ace alloying is now finding practical applications. Cobalt is an important alloying element when enhancement of properties at elevated temperature is the aim. Cobalt dissolved into iroo hardens the alloy, as well as increasing the high temperature performance of the alloy. Therefore, cobalt was selected as the alloying element for this study in which the aim was to obtain a new surface alloy exhibiting good high temperature properties.

2. Experimental details

The substrate material was an alloy steel with chemical composition 0.213 wt.% carbon, 1.462 wt.% chromium, 3.520 wt.% nickel, 0.803 wt.% tungsten, 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD! 0924-0 ! 36(95)02117-5

0.306 wt.% silicon, 0.489 wt.% manganese, and 0.026 wt.% sulphur. The sample was available in tile fornl of slabs 15 mm thick, 80 mm wide and 150 mm long. Chemically pure 200 mesh cobalt powder was used. A precoating layer of cobalt powder was Ibrmed on the surface of tile sample by flame spraying. Tile thickness of tile precoat was ill tile range 0,05 0,!5 ram. Tile alloying treatment was done using a high power CW carbon dioxide laser. The laser power was 1400 W and tile laser beam diameter was 3 ram; laser scanning rates employed were !50, 200, 250, 300, 350, 400 ram/mix. The overlap between consecutive alloying passes was 40"/,,, and for every scanning speed, six passes on the surl~tce of the sample were made. After surface alloy° ing, the specimens were sectioned and polished, then etched with 4% nital for metallographic examination. The microstructural features of the alloyed zone were observed using a model AMRAY 1000B scanning electron microscope. The chemical composition and its distribution in the alloyed zone were measured with a model QX 2000 energy dispersive X-ray spectrometer. The microhardness at elevated temperature was measured using a model HM-100 high temperalure microhardness tester. A loading of 0.I kg was applied with a loading time of l0 s and a holding time of 5 rain at the given temperature. The thermal cycle Cor fatigue test was as follows: heating of the sample to 800 °C, holding for 3 rain, quenching in water at room temperature. The cycle was repeated until cracking was seen on the surface of the sample.

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3. Experimental resultq The depth of the alloyed zone was in the range 0,45-0,60 ram; the depth increased slightly with reduced scanning velocity at constant power and beam diameter, as shown in Fig, 1, Structures in the transitional zone adjacent to the alloyed zone arc shown in Fig, 2, Structures in the zone alloyed with cobalt are shown in Fig. 3(a, b), Observation reveals the presence of some carbide particles in the cobaltized zone, as shown in Fig. 4. The average cobalt content in the surface alloy was in the range 3,10-5,63 wt.%; the maximum cobalt content attained for the given thickness (0,10 ram) of precoated cobalt layer was 9,96 wt,%. The concentration of cobalt varies with laser scanning velocity. Fig. 5 shows the relationship of the average cobalt content with scanning velocity, Tile corresponding microstructures are shown in Fig, 6(a-d). The variation of cobalt concentration with depth in the alloyed zone is shown in Fig. 7. The results of X-ray diffraction experiments confirm that the main phase constitutions in the materials are

not greatly changed by the alloying treatment. Tile main phase in the substrat¢ is alpha-iron + F%C + Cr3Ca + WC + residual austenite, But, after laser alloying, residual austenite is not found in the surface alloy. After alloying with cobalt the microhardness of tile surface at room temperature was between 3200 and 3500 MPa. Moreover, the microhardness at elevated temperature was 2600 MPa at 500 °C, 1900 MPa at 600 °C, and 880 MPa at 700 °C. In addition, the result of the thermal fatigue test showed that the fatigue performance of the treated surface increases to 160% of that of the substrate material.

4, Discussion For a given laser power, cobalt content of the surface alloy can be controlled to some degree. Cobalt content in the surface alloy will increase with increasing scanning velocity, but this increment is limited, because the depth of the alloyed zone will reduce with an increase of scanning velocity. Moreover, if the scanning velocity

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Fig. 3. Microstructures of the zone alloyed with cobalt. is high, there is a possibility of not melting the precoated layer. On the other hand, when the scanning velocity is less than 250 mm/min, the distribution of cobalt in the surface alloy can be homogeneous. When the scanning velocity is greater than 350 mm/ min, the distribution may be heterogeneous. Therefore, scanning velocity is a major factor affecting the

Fig. 4. Carbide particles in the surl'ace alloyed zone.

cobalt o~tent (~) 5.0 4.0 3.0

homogeneity of cobalt distribution in the surface. For laser alloying with cobalt, it is easy to achieve a cobalt conlent in the surface of 5'¼, or so. Although the time of alloying is very short, only about 0.451.20 s, the surface of this material can be effectively alloyed with cobalt. This is one of the advantages of laser surface alloying technology. After laser alloying with cobalt, the performance of the surface alloy at high temperature will be greatly improved because a large number of cobalt atoms dissolve into the iron solution. After alloying with cobalt, no residual austenite exists in the surface alloy. This means that the addition of cobalt eattscs ~he austenite at high temperature to become unstable, and that this austenite changes easily during cooling. It should be pointed out that the improvement in the performance of the iron alloy at high tempera° tare, through ;flloying with cobalt, is iimited~ i|lthough cobalt atoms can greatly strengthen the iron solution. Obviously, surface alloy containing cob~dt should be used only ~vhere it provides particular advantages.

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Fig. 5. Relationship of average cobalt content of the surface alloy with scanning velocity.

114

D#lg Pt,Maa et aL / Journal of Materials Processotg 7k,chnoh~gy 58 (1996) 131-135 '~:~:i'5!iii~i~i~i~i'i~:~i~/~i:,'~~I~I!III~I!I~I~I:I!I ii~ ~:~!!~!~!~!~i~i~@~!~i~!!~!~!~:~i~!~i~:~J~i~::~:~:~iv i!:ii~i::i~:~'i,~i~i~:~!~i!~i!i~!ii'~!i~i~!i:!~!~i ,!!ii~iU~,~!~ii~ii~i/~iii~il i~,~i~i~ ,~ ~i~i!~:,:~ ~,~,~

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Fig, 7, V~ri~ltionof the ~ob~dt content of tile surt't~cealloy with depth in tile zone. 5. Conclusions (1) Laser surface alloying on 0 , 2 1 C - I . 4 6 C r - 3 , 5 N i 0,80W steel has bccn demonstrated, The average cobalt

content of the surface alloy was between 3.10 and 5.63 wt,%. (2) The high temperature hardness at 700 °C of the alloyed surface was 880 MPa, compared with a

substrate hardness of 590 MPa. The flaermal fatigue performance was increased by I~0~,,, when compared with the substrate material.

[~q 17]

References [I] A. Walker. Laser alloying of titanium substrate with carbon and nitrogen, Jourmd of Materials Lelters, 20 119851 9 8 9 - 9 9 5 [2] B.L. Mordike. Properties of laser mtrided surface layers on titanium. Su~:lhce Engha,erhsg, 7 11991 i ! 64 - 173. [3] A.Y. Fasasi, S.K. Roy and A. Galerie. Laser surface alloying of Ti-6AI-4V with silicon Ibr improved hardness and high-temperature oxidation resistance. Materhds Letters, 13 (1992) 204-21 i. [4] P.W. Leech, A.W. Batchelor and G.W. Stachowiak. Laser surIhce alloying of steel wire witll chromium and zirconium. Jourmd o[' ]t,laterhlfi Sciem'e Lc'llers, Ii {1992~ 2345 2351,

[8]

[9]

[10]

[I I]

surface wilh W-V-Co-Cr aHo3mg powder, SEc~7~ce am~ ('~tm~.~ F~'cim~d~gy, 56 1~q92i 47 50. L, Renaud, Corrosion resistance of Fe-Ni-Cr-B laser sur¢~cc allo3s, Sl#?ia c alat ('~almgs T,,chmd