Physical metallurgy of láser surface melted plástic mould steels : a case study

The purpose of this paper is to illustrate the potential of láser surface melting to improve the surface characteristics of plástic mould steels, using a typical plástic mould steel (DIN X43Crl2) as a case study. After láser surface melting the microstructure of this steel is formed by fine dendrites of austenite partially transformed into martensite. Although the equilibrium solidification phase is 8ferrite, the formation of primary austenite is kinetically favoured and this phase tends to predomínate at the high solidification speeds used in láser processing. It was observed that the volume fraction of retained austenite depends critically on the láser processing parameters, so that the microstructure can change from almost completely martensitic to almost completely austenitic by changing the láser processing parameters. Láser melted tool steels show remarkable secondary hardening after tempering at suitable temperatures. In DIN X42Crl3 the secondary hardening peak temperature after LSM (600 °C) is 100 °C higher than after conventioñal heat treatment (500 °C), due to the presence of large amounts of retained austenite. It was observed that this phase only destabilizes above 600 °C, due to the precipitation of M7C3 and stress relieving. After destabilization, retained austenite transforms into martensite during cooling. Secondary hardening is due to the transformation of retained austenite into martensite and to the precipitation of M7C3 and M23C6 carbides.


INTRODUCTION
Mould steels are supplied either in the pretreated or in the annealed state.In the first case, the microstructure is formed of tempered martensite.The tempering temperature is such that a sensible compromise between hardness, wear resistance, toughness and machinability is obtained; so these steels are not heat treated after machining.By contrast, annealed steels such as DIN X42Crl3 present a soft microstructure formed of spherodized carbides dispersed in a ferrite matrix, and are quenched and tempered after machining.
Consequently, the steel can be heat treated to higher hardness than the hardness of pretreated steels allowing longer lifetimes and better service behaviour to be achieved.Láser surface treatment is particularly well adapted to mould manufacturing, because it often improves significantly surface properties without degradation of bulk characteristics or excessive distortion (1)(2)(3).Also, different áreas of the same tool may be treated differently to tailor the surface properties to local requirements.Tool steels can be láser hardened by martensitic transformation, leading to a microstructure and hardness similar to those resulting from induction hardening.Since no melting occurs, the process is very energy efficient and distortion is minimal.Nevertheless, due to the short heat treating cycle, large carbide particles cannot dissolve completely and since the hardness of martensite is roughly proportional to its carbón content, hardening may be insufficient.Therefore, láser hardening is only effective in pre-hardened steels or in normalized carbón and low alloy steels.Conversely, processes involving láser surface melting (LSM) lead to complete dissolution of alloy carbides.Due to the high solidification rates involved, the resulting microstructures are fine and frequently contain non-equilibrium phases and supersaturated solid solutions with useful properties (1), including high hardness and good wear and corrosión resistance (4)(5)(6).Also, láser surface treatment is particularly well suited to small área treatment, a possibility that does not exist with most other surface engineering methods, but extended área coverage is easily accomplished as well by overlapping láser melted tracks.
The properties of láser melted tool steels depend on the solidification microstructure and on the transformations of solidification phases during cooling down to room temperature.The purpose of this paper is to illustrate the results that can be obtained by C0 2 láser surface melting of a plástic mould steel.The correlation between microstructure, properties and processing parameters will be clarified and the principies which govern the formation of microstructure explained.A typical plástic mould steel (DIN X42Crl3) will be used as an example.This material (chemical composition indicated in table I) is supplied in the annealed condition, with a microstructure consisting of M 2 3C 6 and M 7 C 3 carbides dispersed in a matrix of (-ferrite and with a hardness of 250 HV0.2.A detailed description of these results was reported previously (1 and 7-10).

MICROSTRUCTURE OF THE LÁSER MELTED MATERIAL
According to the C-Cr-Fe phase diagram (11), the solidification of a Fe-0.47C-13Crsteel should start with the formation of 8-ferrite, followed by the monovariant peritectic reaction L+8->7 and, finally, by the quasi-peritectic reaction L+8^7+M 7 C 3 , but the solidification microstructures show that austenite is the primary solidification phase (1).This discrepancy has recently been explained on the basis of kinetic arguments (7 and 8): under constrained growth conditions, once nucleated, the phase which presents the máximum interface temperature is kinetically favoured (12).In the present case, the solid below the melt pool provides interfaces for epitaxial growth of 8 and 7, so there is no nucleation barrier and the phase which grows with higher dendrite tip temperature will predomínate.Comparison of the calculated dendrite tip temperatures of 7 and 8 as a function of the solidification rate in the range 10 3 to 2 x 10 _1 ms _1 show that this temperature is higher for 7 than for 8 in all the solidification speed range (7 and 8).In practice this means that the equilibrium primary solidification mode will never be observed in láser surface melting of DIN X42Crl3.
The microstructure of the transverse cross section of a láser melted DIN X42Crl3 steel is shown in figure 1(1).Under the melted layer a heat affected zone (HAZ) is observed, where only solid state transformations occur.The HAZ and the melted layer are separated by a transition zone (TZ) where liquid and solid coexisted during the láser treatment.The thickness of the transition zone will be significant when the thermal gradient is low and the liquidus-solidus temperature interval large, and in this case it may negatively affect the mechanical behaviour of the material.The melted layer is formed by a plañe front solidification zone (PFSZ), stabilized near the bottom of the melted layer due to the large G/R ratio prevailing during the first stages of solidification, and a cellular-dendritic zone   (CDZ).The microstructure of the cellular-dendritic zone is formed by fine austenite dendrites partially transfomed to martensite.The primary arm dendritic spacing is within the range 1 to 10 mm for the láser treatment parameters used, which is in good agreement with the valúes predicted by solidification theory models (8).Transmission electrón microscopy observations showed that martensite is mainly lath-type, though some twinned martensite was also observed and that both martensite and austenite presents a very high dislocation density (1).X-ray diffraction analysis showed that the proportion of austenite which transforms to martensite increases with decreasing cooling rate, Le., with increasing láser power and decreasing scanning speed (9).This behaviour is illustrated in the figures 2 a and 2 b, which show the microstructures of samples treated with the same láser power (P = 2.500 W) and different scanning speeds.The sample treated at 1 mms" 1 is almost completely martensitic, while the sample treated at 80 mms-1 is almost completely austenitic.The parametric dependence of the microstructure of láser surface melted DIN X42Crl3 is shown in figure 3 (9).The mean hardness of the láser melted región varíes between 400 and 700 HV, depending on the proportion of retained austenite.

TEMPERING BEHAVIOUR
The presence of large proportions of retained austenite in láser melted tool steels is generally considered to have a detrimental influence on tool steel properties, leading to modérate hardness, unpredictable wear behaviour, risk of cracking during grinding and electrospark machining, and dimensional changes in service (13).The elimination of this phase can be made either by choosing the adequate processing parameters, as previously discussed, or by tempering at a suitable temperature (10).The tempering treatment has the additional advantage of further increasing the hardness of the steel by secondary hardening.Samples of láser surface melted DIN X42Crl3, with approximately 60 % of retained austenite, were submitted to isochronal tempering treatments in a temperature range between 400 and 700 °C.In  FIG.3.-Dependencia de la microestructura con los parámetros de la fusión superficial mediante láser para el acero DINX42Crl3 (10).
figure 4, the tempering curve láser surface melted samples is compared with the tempering curve of the same material after conventional quenching from an austenitization temperature of 1,025 °C (10).It can be observed that the secondary hardening peak temperatures and hardness are 600 °C and 620 HV for láser processed material compared to 500 °C and 570 HV for the same material after conventional quenching.
In order to understand the causes of the shift for higher temperatures of the secondary hardening peak, sample analysis by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electrón microscopy (TEM) were made (10).It was concluded that the shift in the secondary hardening peak is closely related to the presence of large proportions of retained austenite in the láser melted material.It was observed that austenite retained in láser melted steels differs from the austenite retained after conventional quenching because, while the latter decomposes during tempering in the 200-350 °C temperature range for low alloy steels or is destabilized by holding at 500 °C for high alloy steels (14), the former only transforms after tempering at 600 °C.DSC and TEM results showed that the destabilization of austenite retained after láser surface melting occurs only after partial relieving of residual stresses and precipitation of M 7 C 3 within this phase, leading to a decrease in the solute atoms segregated at dislocations and M/7 interfaces.After destabilization, retained austenite transforms to martensite during cooling if the critical cooling rate is exceeded or to ferrite and carbides if the cooling rate is lower.In láser surface melted steel both the carbide precipitation and the transformation of austenite into martensite are responsible for the increase in hardness observed at 600 °C.It is interesting to note that, whereas in conventionally heat treated steels, secondary hardening is mainly due to dispersión hardening by the alloy carbides, in LSM steels the secondary hardening results from the larger extensión of the 7^M transformation as well as precipitation reactions within austenite and martensite.This may explain the higher hardness of the secondary hardened steel after LSM as compared with the secondary hardness peak after conventional treatment (620 and 570 HV, respectively).Figure 5 resumes the main differences between the transformation during tempering after conventionally quenching and after láser surface melting.

TABLE L -
Chemical composition of the DIN X42Crl3 steel.