Effect of laser melting processing on the microstructure and wear resistance of gray cast iron Lian Shen and Chenglao Departmmt of Materials 710049 @3binaj
Science and Engineering,
Xi’an Jiaotang University,
(Received October 11, 1990; revised and accepted January 7, 1991)
Abstract The microst~ct~es of the laser-hardened layer on gray cast iron were examined by optical microscopy, transmission electron microscopy and scanning electron microscopy. The wear test was done in a sliding wear device. The experimental results show that a structure with dendritic (M+A’) and interdendritically laminal transformed ledeburite (M + A’ + Fe3C) is formed in the melted layer and a structure with clusters of acicular martensite, retained austenite and graphite, i.e. (M +A’ + G), is formed in the solid transformed layer after laser melting processing. The martensite is a mixture of dislocation martensite and twin martensite. Dislocation pileups and twins are found to exist in the retained austenite. The strain-induced martensite transformation occurs in the retained austenite and the sequence of the transformation process during sliding wear is retained austenite (f.c.c.) +stacking fault (h.c.p.)-,martensite @.c.c.). The wear resistance of gray cast iron after laser melting processing is remarkably enhanced and the wear process consists of plastic deformation + initiation of microcracks -+ growth of microcracks -+ fracture (exfoliating metal chips) due to eliminating graphites (microcracks). The formation of transformed ledeburite and a large amount of retained austenite, the elimination of graphite flakes, and refinement of the microstructure are beneficial to the improvement in wear resistance after laser melting processing.
Recently, the enhanced wear resistance of gray cast iron by high-energy laser beams and the microst~cture in the laser-processed layer have been studied [l-4]. The type, shape and dist~bution of transformation products during laser processing and the effect of the transformation products on wear resistance have been studied [5, 61, but until now the phase transformations during wear have not. In this paper, the microstructure, the effect of microst~cture on wear resistance, the strap-induced martensite transformation and the reasons for enhanced wear resistance by laser melting processing are studied. 2. Experimental
and heat tr~at~t HTZO-40 gray cast iron was used in the test; its chemical composition is 3.3. wt.% C, 2.3 wt.% Si, 1.1 wt.% Mn, 0.25 wt.% P, 0.35 wt.% S. 0 Eisevicr Sequoi~r~~ed
in The Netherlands
The size of the sample was 30 X 16 X 10 mm3. Some of the samples were prelimin~ly coated with manganese phosphate to improve absorption of laser radiation and were processed by a GJ-1 type I.2 kW CO, continuous wave laser device to obtain a suitable depth of melted layer on the surface of the samples. The parameters used for the laser melting processing were 950 W power, 4 mm beam diameter, 2’7.5 mm focus and 21 mm s-’ scanning speed; the other samples were processed directly by high-frequency-induced heating, quenching in water containing sodium nitrate and tempering for 30 min at 180 “C.
The microstructures were observed by an MM3 optical microscope. The thin foil was prepared in an ionic micromilling instrument and the morphology and sub-structures of the phases were examined by transmission electron microscopy (TEM) (,JEM-BOOCX). 2.3. Analysis of wear behavior and wear suy$ace ~o~hol~g~ The gray cast iron HT20-40 is a material used for making cylinder jackets. To simulate the working conditions of a cylinder jacket, the sliding wear test was done using the apparatus shown in Fig. 1. The Iaser-hardened and induction-hardened samples, the upper sample in Fig. 1, were respectively worn with the matching sample, the lower sample in Fig. 1, which was made of wear-resistant cast iron containing Cr-MO-Cu alloy elements under constant load (P= 19.6 N). The sizes of the wear surfaces were respectively 30 X 16 mm2 and 180 x 20 mm2 for the upper and lower samples. The load of 19.6 N exerted a pressure of 0.51 MPa on the wear surface, Engine oil containing 0.2% SiC abrasive with a particle size of 5-10 pm was used for the lubricant. After wearing-in for 20 h, the weight loss of the sample was measured every 10 h during wear by a TG328A photoelectrically analytical balance. The surfaces were polished for observing the microstructure, perpendicular to the wear direction and at an angle of 15” with the wear surface so that the corresponding relation between the morphology of the wear surface and the microstructure of the polished surface could be observed.
Fig. 1. Schematic
of the sliding wear device.
2.4. Measur-t of retained au&mite befwe and qfkr wear The amounts of retained austenite were measured by a Wmax-III X-ray diffractometer and the wear depth was measured by screw-microruler every 10 h under a constant load of 68.6 N which exerted a pressure of 1.78 MPa on the wear surface during wear in order to obtain the distribution of retained austenite along the depth of the laser-hardened layer after wear. The distribution of retained austenite along the depth of the laser-hardened layer before wear was obtained by X-ray diffraction and chemical etching.
3. Results of the experiments 3.1. Microstructure
The original microstructure of the HTZO-40 gray cast iron consists of a pearlite matrix with graphite flakes, a little ferrite and phosphoric eutectic, as shown in Fig. 2. The microstructure of the laser melting processed layer of HT20-40 gray cast iron is shown in Fig. 3. The surface layer is a melted layer and the subsurface layer is a solid transformed layer. TEM observations on a thin foil cut from the melted layer show that the microstructure consists of extremely fine dendritic (M +A’) and ~terdend~tically laminar transformed ledeburite (M +A’ + Fe3C), in which the martensite is a mixture of dislocation martensite and twin martensite. The martensite plate is extremely fine, about 0.3-0.6 pm in length. Dislocation pile-ups are found to exist in the retained austenite as shown in Fig. 4. The microstructure is non-uniform within the solid transformed layer as shown in Fig. 3; the microstructure near the melted layer consists of clusters of acicular martensite and a large amount of retained austenite, and the microstructure away from the melted layer consists of clusters of martensite, retained austenite and a large amount of graphite. The length of the martensite is different, near the graphite it is about 6-9 pm and within the clusters of acicular martensite it is about
Fig. 2. Original mi~ros~ct~e
of HT20-40 gray cast iron (original
Fig. 3. Microstructure of the laser melting processed magnification X 500).
layer of HTZO-40 gray cast iron (original
l-4 pm. TEM observations show that the martensite in the clusters of acicular martensite is also a mixture of dislocation martensite and twin martensite, and dislocation pile-ups and twins are also found in the retained austenite as shown in Fig. 4. The microstructure in the high-frequency-induced hardened layer of HT20-40 gray cast iron consists of indistinct acicular martensite, retained austenite, phosphoric eutectic and a large amount of graphite, as shown in Fig. 5.
3.2. Wear behavior and morphology of wear surface Plots of the dependence of weight loss on wear time are shown in Fig. 6. The weight loss of laser melting processed sample is much less than that of the high-frequency-induced processed sample, Le. the wear resistance of laser melting processed gray cast iron is obviously higher than that of the induction processed iron. The correspondence between the morphology of the wear surface and the microstructure observed by SEM is shown in Fig. 7. The wear scar is straight and there are less exfoliation pits on the wear surface after melting processing (Fig. 7(a)), but the wear scar is distorted and there are more exfoliation pits on the wear surface after induction processing (Fig. 7(b)). 3.3. Change in retained austenite during wear The curves of the distribution of retained austenite along the depth of the hardened layer for laser melting processing before and after wear are shown in Fig. 8. The amount of retained austenite decreases by about 4%-l 1%
Fig. 4. TEM images of the laser-melted layer and solid transformed layer of HTZO-40 gray cast iron: (a) bright field micrograph of dendritic (M +A’) in me&cd layer (original magnification X 37 000); (b) bright field micrograph of interdcndritically transformed ledeburite (M + A’ + Fe3C) (original magnification X 20 000) white lamina showing Fe3C dark lamina showing M +A’; (c) bright field micrograph of dislocation pile-ups in retained austcnite (A’) (original magnification X88 000); (d) bright field micrograph of cluster of martensite in solid transformed layer (original magnification X 20 000).
during wear. TEM observations of the thin foil cut from the wear surface show that the dislocation density is increased, dislocation ceUs and a lot of stacking faults are formed in the retained austenite and some of the retained austenite is induced to transform into acicular and lathy martensites. The crystal structures of the two kinds of martensite are all b.c.c. lattice type as shown by the electron dif!fraction patterns in Figs. 9 and 10, The sequence of the strain-induced transformation of martensite is retained austenite (f.c.c.) --f stacking fault (h.c.p.) -+ martensite (b.c.c.), and the lattice relation between martensite and austenite is (111) f.c.c. ll(110) b.c.c. which corresponds to the K-S relation as shown in Fig. 11. 4. Discussion 4.1. The fortes of microstructure duri~ laser m&ing processing The laser-hardened layer of HT20-40 gray cast iron consists of a melted layer and a solid t~nsfo~ed layer. The surface of the gray cast iron melts
Fig. 5. Microstructure (original magnification Fig. 6. Dependence induced processing
of high-frequency-induc X 500).
laysx of HT20-40
of weight loss on wear time of HT20-40 gray cast iron after high freq luenc :Y (0) and laser melting pro1“es.sing (CI).
Fig. 7. SEM images of correspondence between the morphology of the wear surface and microstructure in layer of HT20-40 gray cast iron after wear for 30 h (original magnitication x 1000); (a) laser melting processed sample; (b) high-frequency-induced process sample.
quickly and the graphites and phosphoric eutectic existing in the original structure are completely dissolved in the laser-melted zone under the processing conditions of high power input at a slow scanning speed. The nucleation rate is much faster than the growth rate during the rapid solidification owing
1 40 . 30 0
IO t OO-
100 im -
Fig. 8. Distribution of retained (0) and after (0) wear.
along the depth of the laser hardened
Fig. 9. TEM images of strain-induced (a) bright area micrograph (original micrograph.
acicular martensite in retained austenite during wear: magnification X 73 000); @> surface area diffraction
to the high cooling rate and supercooling, so the extremely fine microstructure and ~croc~stall~e structure are obtained. The primary austenite grows in dendritic form owing to a large consitution~ supercoormg zone in front of
Fig. 10. TEM images of sprat-~nduccd lathy martensite in retained austcnitc; (a) bright field micrograph (original magni~cation x 30 000); (b) dark Geld micrograph (original magni~cation x30 000); (c) surface area diffraction micrograph.
the interface between the liquid and solid. The interdendritic ledeburite is the eutectic consisting of very fine laminar cementite and austenite . The austenite is partly transfo~ed into martensite during rapid cooling, so that the transformed hypoeutectic structure is composed of fine dendritic (M +A’) and interdendritically laminar transformed ledeburite (M +A’ + Fe3C). Dislocation pile-ups and twins are found to exist in the retained austenite because the martensitic transformation results in strong plastic strain in the neighboring austenite due to the volume expansion and low stacking fault energy. The m~~rost~cture in the solid transformed layer is non-upon because the austenization temperature is different owing to the negative temperature gradient along the hardened depth. The rate of austenization is fast and the majority of the graphite dissolves in the austenite; the microstructure consists of clusters of martensite and considerable retained austenite in the solid transformed layer beneath the melted zone, because the temperature near the melted layer is very high and approaches the melting temperature of the gray cast iron. Microsegregation in the austenite is obvious because of the short heating time and the slow diffusion rate of carbon and alloy elements during laser processing. The
Cd) Fig. 11. TEM images
of strain-induced martensite transformation: (a) bright field micrograph of stacking fault in retained austenite (original magnification X 30 000); (b) bright field micrograph of strain-induced martensite (original magnification X 73 000); (c) dark field micro~aph of scan-educed martensite (original ma~cation X 73 000); Cd) surface area diffraction of retained austenite, stacking fault and strain-induced marten&e in retained austenite during wear.
austenite near the graphite has a high carbon content and its MS (starting temperature of martensite transformation in undercooling austenite during cooling) temperature is low. It partly transforms into martensite and most of it is retained after rapid cooling. The austenite far from the graphite has a low carbon content and transforms into clusters of fine acicular martensites
consisting of a mixture of dislocation martensite and twin martensite. Because the temperature gradually decreases along the hardened depth, the dissolving rate of graphite is slowed down and the majority of graphite is retained. The microstructure consists of clusters of martensites, retained austenite; the sizes of the graphite flakes and the martensite, and the amount of retained austenite, decrease gradually along the hardened depth in the soiid transformed layer far from the melted zone. 4.2. Strain-induced austenite
in the retained
The retained austenite in the laser-hardened layer of gray cast iron is different from the austenite in austenite stainless steel and high manganese austenite steel. It is an undercooling austenite which is thermodynamically unstable. The strain-induced martensite transformation can occur in the retained austenite under a lower stress. During wear, plastic deformation occurs and stacking fault or E’-martensite (h.c.p.) is produced in the retained austenite owing to low stacking fault energy and the formation of extended dislocations, i.e. a/2(10i)--+a/6(112)+a/6(2ii)+stackingfault
Therefore, the diffraction contrast in Fig. I1 is very bright in the stacking fault zone. The martensite nucleates in the stacking fault zone and the sequence of the strain-induced transformation of martensite is retained austenite (f.c.c.) + stacking fault (h.c.p.) -+ martensite (b.c.c.), i.e. the dislocation of b =u/2( IOi) dissolves into two partial dislocations of b = a/6( 112) and b = a/6(2i i> on every three parallel (111) planes. These partial dislocations move along the (11 I} planes and the stacking fault zones of a certain thickness are formed; b.c.c. martensite is formed by strain along b = a/l 2( 112) on the invariant plane and expansion and compression along the axial direction. The lattice relation between martensite @.c.c.) and retained austenite (f.c.c.) is (I 11) f.c.c. tl(1 IO) b.c.c. which corresponds to the K-S model. This result supports the model of lattice transfo~ation proposed by Dash [S]. 4.3. The reason for processing
by laser melting
First, transformed ledeburite with high hardness is formed and the microstructure is refined during laser melting processing. The ledeburite consists of M, A’ and Fe3C, the Fe3C being a major strengthening phase because its microhardness is very high, especially when it is in fine lamina f7]. The fine laminar Fe3C and fine martensite can decrease the brittleness and enhance plastic deformation resistance and are beneficial to wear resistance. Second, the graphites, i.e. microcracks, are eliminated in the laser melted layer (Fig. 7(a)). It is necessary to initiate microcracks at the areas where there is a concentration of stress caused by plastic deformation during wear. The microcrack initiation needs to absorb a great amount of energy
and is beneficial to enhanced wear resistance. The sequence of the wear process is plastic deformation+initiation of microcracks *growth of microcracks + fracture (exfoliating metal chips). When there are a large amount of graphite particles in the high-frequency-induced hardened layer, the graphite particles break away from the metal matrix and their sites become microcracks (Fig. 7(b)). The microcracks grow to a critical size and conjugate together producing exfoliating metal chips and decreasing wear resistance. The sequence of the wear process is growth of microcracks + fracture (exfoliating metal chips). There are a lot of exfoliation pits at the sites of graphite particles on the wear surface and it is shown that the graphite particles decrease the wear resistance. Third, a large amount of austenite is formed in the laser-hardened layer during laser melting processing. The plastic deformation, work hardening and strain-induced martensite transformation occur in the retained austenite during wear. These processes help decrease the rate of initiation and growth of microcracks and enhance the wear resistance owing to the absorbtion of energy and relaxation of stress concentrations during these processes.
5. Conclusion (1) The laser melting processed layer of HT20-40 gray cast iron includes a melted layer and a solid transformed layer. The microstructure in the melted layer consists of dendritic (M +A’) and interdendritically laminar transformed ledeburite (M +A’ + Fe3C). The microstructure in the solid transformed layer consists of clusters of acicular martensite, retained austenite and graphite, i.e. (M +A’ + G). The martensite in the laser-hardened layer is a mixture of dislocation martensite and twin martensite. The microstructure in the high-frequency-induced hardened layer of HT20-40 gray cast iron consists of indistinct acicular martensite, retained austenite, phosphoric eutectic and a large amount of graphite. (2) The retained austenite in the laser-hardened layer of HT20-40 gray cast iron is unstable. The dislocation density increases, a large number of dislocation cells and stacking fault zones form and a strain-induced martensite transformation to form acicular and lathy martensites occurs in the retained austenite during wear; these changes are beneficial to enhanced wear resistance. The crystal structure of the strain-induced acicular and lathy martensites is b.c.c. The sequence of the strain-induced martensite transformation process is retained austenite (f.c.c.) --) stacking fault (h.c.p.) --f martensite @.c.c.), and the lattice relation between martensite and austenite is (111) f.c.c. Il(ll0) b.c.c. which corresponds to the K-S model. (3) The wear resistance of HT20-40 gray cast iron is obviously enhanced after laser melting processing, the reasons are the formation of high hardness transformed ledeburite, a refined microstructure, the elimination of graphite and the formation of a large amount of retained austenite.
(4) The sequences of the wear process of HTZO-40 gray cast iron are plastic deformation + initiation of microcracks 3 growth of microcracks + fracture (exfoliating metal chips) after laser melting processing and growth of microcracks --) fracture (exfoliating metal chips) after high-frequency-induced hardening processing.
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