Improvement of wear and hot melt loss resistance of metal carbide layers on H13 steel

In this paper, H13 steel was pre-carburized. Then niobizing and vanadizing layers were prepared by pack cementation method. The high temperature friction and wear properties and hot melt loss properties of different layers and substrates were studied by microhardness tester, metallographic microscope, scanning electron microscope, energy dispersive spectrometer, high temperature friction and wear tester, optical profilometer and Raman spectrometer. The results show that the thickness of the vanadizing layer is 12.7 μm, and the microhardness of the niobizing layer and the vanadizing layer is close, which is about 5 times that of the matrix. The lowest wear rate at 500 °C of the vanadizing layer is 1.03 , which is about 1/6 of the matrix. The vanadizing layer and niobiumizing layer can effectively reduce the friction coefficient, greatly improve the surface hardness and wear resistance of H13 steel, and prolong its service life. The comprehensive performance of vanadizing layer is the best. The vanadizing and niobiumizing treatment can significantly improve hot melt loss resistance of H13 steel and it can be used to prolong the serving life of hot die casting mold for Al.

thermal reaction diffusion technology, and compared the properties of niobizing layers prepared by the three steels.The corrosion resistance of niobium carbide coating prepared by D2 steel is the best according to the potential polarization curve and EIS spectrum.Su W et al [17] prepared Cr 2 O 3 -BaCrO 4 coating on ceramic surface by plasma spraying method, and explored the friction and wear characteristics of Cr 2 O 3 -BaCrO 4 composite coating at 25 °C ∼ 700 °C.It is found that the Cr 2 O 3 -BaCrO 4 coating has a dense layered structure.Above 400 °C, the Cr 2 O 3 -BaCrO 4 coating will form BaCrO 4 on the wear surface.BaCrO 4 is a high temperature solid lubricant, which can significantly improve the wear resistance of the ceramic surface at high temperature.
The vanadizing and niobiumizing treatment by powder pack cementation method is simple and easy to operate, and mixing agent can be reused.The layer prepared by vanadizing and niobiumizing layer are mainly composed of VC x and Nb x and the hardness is improved.The wear resistance at high temperature and hot melt loss resistance for Al are further studied to improve the service life of hot die steel.

Experimental materials and methods
The H13 steel used in present work is treated by quenching and tempering (QT).The specific chemical composition (wt%) was detected by the Q4 direct reading spectrometer of Bruker Company in Germany, as shown in table 1.The H13 steel was cut into a sample with a size of Φ15 mm × 4 mm, and the Φ15 mm surface was used as the sample surface in the experiment.Firstly, H13 steel was pre-carburized, and then vanadizing and niobizing were carried out on H13 steel by pack cementation method, the infiltration agent is composed of 5% NH 4 Cl, 47.5% Al 2 O 3 , 47.5% V-Fe, 5% NH 4 Cl, 47.5% Al 2 O 3 , 47.5% Nb-Fe, which are fully mixed and poured into the infiltration tank.The infiltration tank is placed in a box-type resistance furnace with a crucible clamp, and the heating temperature is 940 °C and the holding time is 7 h, after the heat preservation, the pot was taken out with a crucible clamp, cooled in the air, and the sample was taken out after the pot was cooled to room temperature.The morphology observation, microstructure analysis and performance test of different layers were carried out.The friction and wear test and hot melt loss test were carried out on the matrix and different layers.
The microstructure was analyzed by Axio Vert.21Al metallographic microscope.The hardness of the layer was measured by HVS-1000 automatic microhardness tester.The microstructure and element distribution of the samples were analyzed by Sigma500 scanning electron microscope (SEM) and NORAN-QUEST II energy spectrometer.The friction and wear test of the sample was carried out by using the UMT-3 high temperature friction and wear tester.The friction and wear experiments were repeated three times at the same wear parameter, and the results were compared and analyzed.The friction pair was Φ6.35 mm Al 2 O 3 ceramic ball, the experimental temperature is 500 °C, the time is 20 min, the rotation radius is 5 mm, the sliding distance is 32 m, the normal load is 10N, and the friction speed is 0.256 m s −1 , and the friction coefficient was compared.The surface morphology of the sample after wear was observed using a MicroXAM-800 non-contact optical profiler and the volume of wear was measured to calculate the volume wear rate.The Raman spectrum of the wear scar after high temperature friction and wear was measured by Thermo DXR2xi Raman spectrometer.The wavelength of the light source was 785 nm to determine the composition of the wear scar after friction and wear.The corrosion behavior in molten aluminum of H13 steel with different treatment (QT, niobizing and vanadizing) was studied by static hot melt loss test.The samples after different treatment (QT, niobizing and vanadizing) were kept in molten Al at 700 °C for 40 min.Then the differences in the thermal melting loss in mass and subsurface morphology of the three samples were compared [18].

Experimental results and analysis
3.1.Cross-section morphology and hardness analysis of different layers Figure 1 shows the cross-sectional morphology of H13 steel with different treatment (QT, niobizing and vanadizing).It can be seen from that the niobizing layer and vanadizing layer prepared by pack cementation are uniform, continuous and dense.The right axis of figure 2 shows the thickness of the vanadizing layer, and it can be seen that the thickness of the vanadizing layer is relatively high at 12.7 μm. Figure 2 left axis is the microhardness of H13 steel with different treatment (QT, niobizing and vanadizing).The microhardness of each sample is measured 7 times, and the average is taken after removing a maximum and minimum values.It can be According to [19], the wear volume in Formula 1.1 is to approximately regard the wear area generated by the friction between the sample and the dual ball as a spherical shape, and to find the wear volume of the sample at a certain position in the spherical area.V is the wear volume (mm 3 ), R 1 is the dual sphere radius (mm), R 2 is the friction radius (mm), S is the wear mark width (mm), L is the sliding distance (mm), n is the motor speed (rpm), T is time (s), F is the normal load (N), δ Is the volume wear rate (mm 3 /N•m).The dynamic friction coefficient  and average friction coefficient curves are obtained by repeated tests, which can only evaluate the time and sliding distance of the sample to be stable.The standard for evaluating the wear resistance of the sample is the wear rate, and the wear resistance of the sample with low wear rate is better.It can be seen that the volume wear rate of different metallizing layers is quite lower than that of QT treatment, the wear resistance of QT is obviously poor.After different metalizing treatments, the wear resistance of H13 steel can be significantly improved, thereby prolonging its service life.The volume wear rate of the matrix after vanadizing treatment is the smallest, and its wear resistance is the best.

Morphology of worn scars
Figure 6 shows the three-dimensional worn surface topography of H13 steel with different treatment (QT, niobizing and vanadizing).As can be seen from figure 6(a), the QT treated H13 steel surface wear is serious and the wear depth is about 14 μm.The accumulation occurs at the sliding edge and it is indicated that the steel has weak softening resistance at high temperature and plastic deformation occurs mainly during the wear process.Figure 6(b) shows the topography of niobizing layer after friction and wear.It can be seen that the worn scar is narrow relativity comparing with that of QT and the wear depth is about 4 μm. Figure 6(c) shows the the topography of vanadizing layer after friction and wear.It can be seen that the wear depth is about 3.2 μm, the width of worn scar is only about 0.5 mm.The worn surface zone is smooth.
Figure 7 shows the SEM morphology of worn surfaces after friction and wear of H13 steel with different treatment (QT, niobizing and vanadizing).Figure 7(a) is the worn surface of QT H13 steel.There are obvious furrows and a small amount of white particles on the surface of the substrate, after EDS analysis, the particles contain more oxygen, and the particles are oxides that should be separated from the surface of the wear marks.The magnification of figures 7(b) and 6(b) is different, in figure 7(b) the wear position during the scanning electron microscope test is difficult to correspond to the wear position of figure 6(b), so the two images do not match in surface roughness.Figures 7(b) and (c) are worn surface morphology of niobizing layer and vanadizing layer, respectively.It can be seen from the SEM morphology that the worn surface the diffusion metallizing layer is smooth, and there is a groove morphology parallel to the friction direction.The furrow is shallow, and there is fewer wear debris on the surface.From the worn surface morphologies and composition, we can see that the wear mechanism for QT H13 steel is mainly adhesive wear and oxidative wear.Under grinding effect of ball, plastic deformation occurs and the attached particles break, which is consistent with the description in [20][21][22].The wear mechanism for Niobizing and Vanadizing H13 steel are mainly abrasive wear and oxidative wear and the ploughing morphology is mild.

Composition analysis worn scar surface
Figure 8 shows the Raman spectrum of the surface wear scar after high temperature friction and wear.It can be seen from the figure that the H13 steel (QT) mainly generates Fe 2 O 3 phase on the surface of the wear scar under the action of high temperature and high speed dry friction.The surface generated Fe 2 O 3 has a good anti-friction effect, protects and lubricates the matrix, and makes it have a lower friction coefficient.The Raman characteristic peaks corresponding to 131, 192, 286, 391 and 641 cm −1 in the experimental spectra of vanadium carbide are similar to VO 2 positions, it is determined that the vanadium-permeable layer generates VO 2 through high temperature friction and wear on the surface [23].Metal oxides VO 2 and Nb 2 O 5 are formed on the surface of the vanadizing layer and the niobizing layer during high temperature friction and wear.Transition  metal oxides (such as NbO x and VO x ) undergo plastic deformation rather than brittle fracture at high temperature, the friction coefficient of the oxide decreases with the increase of temperature, these metal oxides have good lubricity at high temperature, -thus effectively reducing the friction coefficient [24,25]

Sectional morphology after liquid aluminum corrosion
Figure 10 shows the cross-sectional morphology and element distribution of H13 steel with different treatment (QT, niobizing and vanadizing) sample after hot melt loss.Figure 10(a) is the cross-sectional morphology of QT after hot melt loss.It can be seen that there is a dense layer of Al element and a large amount of Fe element in the outermost part.Al element and Fe element generate metal compounds.The metal compounds are mainly composed of different iron-aluminum compounds FeAl, FeAl 2 , Fe 2 Al 5 , FeAl 3 , etc, and the metal compounds reach a certain thickness and gradually fall off into the aluminum liquid, resulting in the melting loss of QT H13 steel [26,27].Figure 10(b) shows cross-section morphology of the niobizing sample.The surface of the layer has less melting loss and better hot melting loss resistance.It can be seen from energy spectrum that there is no Al element, because the sample is immersed in a saturated sodium hydroxide solution so that the surface of the aluminum liquid is all reacted.Figure 10(c) shows the cross-section morphology of vanadizing layer.The  vanadizing layer is still uniform and dense after the sample is melted, and the thickness of the vanadizing layer is significantly reduced.

Hot melt loss in weight
The weight loss of H13 steel with different treatment (QT, niobizing and vanadizing) sample in the static hot melt loss experiment is shown in figure 11 It can be seen that the hot melt loss weight of QT, niobizing and vanadizing layers is quite different.The maximum hot melt loss weight of QT is 0.424 g.The niobizing and vanadizing layers can significantly improve the hot melt loss resistance of H13 steel.The minimum hot melt loss weight of the niobizing sample is 0.023 g, and the weight loss of the vanadizing sample is 0.0328 g.It can be seen that the hot melt loss resistance of the niobizing sample is better.

Conclusion
Vanadizing layer and niobizing layer were prepared on H13 steel by pack cementation method.The tribological properties at high temperature and hot melt loss resistance to static aluminum were compared.The mechanism of vanadizing layer and niobizing layer the wear resistance at high temperature and static aluminum hot melt loss resistance were studied.
(1) The friction coefficient of vanadized layer at 500 °C is similar to that of QT treated H13 steel, and volume wear rate is about one sixth of QT treated H13 steel.Among them, the wear resistance of vanadium carbide coating is the best, and the wear resistance of H13 steel substrate is obviously poor.
(2) The carbide coating prepared on the surface can effectively isolate the contact between the aluminum liquid and the aluminum element in the matrix, prevent the formation of the iron-aluminum alloy layer, and improve the hot melt loss resistance of the mold.
(3) Comprehensive comparison shows that structure of vanadizing layer is uniform and dense, with excellent high temperature wear resistance and static aluminum hot melt loss resistance, and has a good application prospect.

Figure 2 .
Figure 2. Microhardness and thickness of different samples.

Figure 3 .
Figure 3.The schematic diagram of friction and wear experiment.

Figure 4 .
Figure 4. Dynamic and average friction coefficient of different samples.

3. 3 .
Hot melt loss in Al 3.3.1.Surface morphology After hot melt loss resistance experiment in static Al, the macroscopic morphology of H13 steel with different treatment (QT, niobizing and vanadizing) is shown in figure9.It can be seen that the surface of H13 steel with different treatment (QT, niobizing and vanadizing) is not uniform after the hot melt loss, and some parts of the metallized sample peel off.It can be seen from figure8(a) that QT sample has serious melting loss and obvious depression on the surface figure 8(b).The surface of the niobizing layer sample is still smooth and bright after corrosion in figure8(c).The surface of the vanadizing layer is relatively flat, and there is no obvious peeling.

Figure 10 .
Figure 10.The SEM morphologies of different cross section samples after melting in Al.(a) Q+T (b) Niobizing (c) Vanadizing.

Figure 11 .
Figure 11.Hot melt loss weight of different samples after melting in Al.
seen that the surface microhardness of H13 steel can be significantly improved by niobizing and vanadizing treatment.The microhardness of vanadizing layer is 2213.8HV 0.2 , and the microhardness of niobizing layer is 2189.6HV 0.2, and the microhardness of QT H13 steel is 465.6 HV 0.2 .The microhardness of the vanadizing layer and the niobizing layer is similar, which is about 5 times of the microhardness of the matrix.3.2.Tribological behavior at high temperature 3.2.1.Friction coefficient and wear rate The friction and wear experiment is shown in figure 3. The dynamic friction coefficient and average friction coefficient of the H13 steel with different treatment (QT, niobizing and vanadizing) are shown in figure 4. It can be seen that the high-temperature friction coefficient of H13 steel with different treatment (QT, niobizing and