Effect of alloy composition on the 45 steel microstructure and properties of scanning electron beam surface alloying

In order to improve the surface properties of 45 steel, the effects of different scanning electron beam treatments on the surface properties of 45 steel were studied. The research results indicate that after scanning electron beam surface alloying (SEBSA) treatment, in addition to forming needle like martensite, carbides (tungsten carbide and molybdenum carbide) also exist in the alloying zone of the strengthening layer, moreover, the grains become finer. After scanning electron beam alloying treatment, the surface hardness reached 1250HV, which is 5-6 times the substrate. Compared with the wear loss of the matrix, the wear resistance of the sample after SEBSA treatment increased by nearly six times.


Introduction
45 steel produces a large amount of scrap due to its low surface hardness and poor wear resistance [1][2][3].Scanning electron beam can effectively change the surface microstructure and improve surface performance, due to its "uniform energy density distribution, sudden heating and rapid cooling" characteristics [3,4].By adding different alloy elements, the alloy layer can form a special phase structure and improve the corresponding surface performance, and improve the disadvantages such as weak adhesion and loose organization [5], SEBSA is used to treat the surface of 45 steel, effectively improving its utilization rate.
At present, many scholars conducted partial research on related issues.Grenadyorov et al. used low energy, high current to study the surface Ti alloying of AISI316L stainless steel.The research results showed that the surface hardness of the sample increased to around 850HV.Singh studied the effect of W, Mo, Cu, and Nb on the surface properties.Jneir et al. investigated the effect of hard particle content on the performance of the alloy layer by directly adding hard particle phases such as WC and electron beam surface treatment, the research results showed that as the content of hard particle phase increased, the surface hardness of the sample gradually increased, when the WC content reached about 40%, the hardness of the alloy layer reached around 1400HV.Barbara et al. further studied the effects of different hard particle phases (WC, TiN, SiC, and Al2O3) on the microstructure and properties of the alloy layer, the research results found that under the same content of hard particle phases, the mechanical properties of the alloy layer did not change much, but the heterogeneous nucleation and grain growth within the alloy layer with hard particle phases had a significant impact.
By comparing the effects of scanning electron beam surface treatment and W and Mo alloying

Material preparation
The experimental substrate is commonly used 45 steel bars after annealing.During the experiment, the bar material was processed into 40 mm using a vertical milling machine × 40 mm × A 40 mm specimen.

Equipment and experimental methods
During the experiment, observe the organizational structure using a Tension metallographic microscope; Use HMV-ZT Vickers microhardness tester for hardness testing.

Comparative analysis of surface hardness
Select one sample from each of the scanning electron beam solidification strengthening treatment and scanning electron beam alloying treatment, as shown in Figure 1, and quenched and tempered state.Conduct surface hardness testing on the selected samples.From Figure 2, it can be seen that the average hardness of the surface of 45 steel in the factory condition (after normalizing treatment) is 230HV.After quenching and tempering treatment, the average hardness of the surface of the sample in the factory condition is about 290HV; After SEB melting treatment, the surface hardness reaches 820HV, which is 2-3 times higher than that of quenching and tempering treatment; after SEB Mo alloying, the surface hardness is 1150HV; after SEB W alloying, the surface hardness reaches 1250HV, which is 4-5 times higher than that of quenching and tempering treatment.

Comparative analysis of surface layer microstructure
From Fig. 3(a), it can be seen that the matrix microstructure of 45 steel is ferrite and pearlite.After scanning electron beam melting treatment, the microstructure of the sample surface is acicular martensite and lath martensite, and the martensite grains are fine, as shown in Fig. 3(b).From Fig. 3(c), it can be seen that after W SEBSA treatment, the microstructure of the surface layer of the sample is needle like martensite and tungsten carbide particles.Fig. 3(d) shows that after Mo SEBSA, the surface layer of the sample is needle like martensite and forms a large amount of molybdenum carbide.
Compared with melt strengthening, the microstructure and grain size of the alloy layer are finer.The generation of carbides is mainly formed by in-situ reactions between carbon atoms, tungsten atoms, and molybdenum atoms during the dynamic melting process on the surface of the sample.Among them, W atoms are more likely to react with carbon atoms than Mo atoms.

Comparative analysis of cross-sectional microhardness and wear loss
The cross-section microhardness of 45 steel samples after different treatments was tested using a microhardness tester, and the results are shown in Fig. 3. From Fig. 4, it can be seen that the hardness of the sample after normalizing the 45 steel substrate is relatively low and remains unchanged at about 230HV.After surface melting treatment by scanning electron beam, the hardness gradually decreases from the surface along the depth direction, and decreases slowly in the hardening zone.Due to the diversity of the structure in the heat affected zone, the hardness decreases rapidly, and remains basically unchanged until the matrix part.The depth of the reinforcement layer reaches 600 μm.The average hardness is 800HV.After SEBSA, the microhardness of the sample surface was greatly improved, reaching 1200-1300HV, and the thickness of the strengthening layer reached 470 μm.The surface hardness of the sample after W SEBSA is slightly higher than that of the sample after Mo SEBSA treatment.Conduct wear resistance experiments on 45 steel samples with different treatments on a friction and wear testing machine, and the experimental results are shown in Fig. 5. From Fig. 5, it can be seen that the matrix of 45 steel after normalizing treatment shows a non-linear increase in wear weight loss with the increase of load.When the load is 50N, the wear weight loss of the sample is 17mg; As the load increased to 150N, the wear weight loss also increased to 58mg.After electron beam melting treatment, with a wear amount of 6.1mg under a load of 50N; When the load increases to 150N, the wear amount is 16.4mg, which is 23% of the wear amount of the untreated sample.After Mo SEBSA of the sample, when the load is 50N, the grinding loss weight is only 3.1mg; When the load increases to 150N, the wear weight loss is 14.5mg, which is 19% of the untreated wear weight loss.Compared with the substrate sample, slightly improved after melting treatment.After W SEBSA, when the load is 50N, the grinding loss weight is only 2.4mg; As the load increases to 150N, the wear loss is 11mg, which is 13% of the untreated wear amount, slightly improved compared to the molybdenum alloying treatment.It can be seen that the wear resistance of the sample after alloying treatment is improved compared to that after melting treatment, and better than that of the untreated sample.

Conclusions
(1) The surface hardness of the 45 steel sample after SEB melting treatment reaches 850HV, after SEBSA, the surface hardness of the sample reached 1250HV, which is 5-6 times the substrate.
(2) After the melting and solidification treatment of 45 steel, the microstructure of the most superficial melting and solidification hardening zone is acicular martensite and lath martensite; After SEBSA, in addition to forming needle like martensite, carbides (tungsten carbide and molybdenum carbide) also exist in the alloying zone of the strengthening layer.Moreover, the grains become finer.
(3) After electron beam melting strengthening treatment and SEBSA, the microhardness of 45 steel shows a non-linear decreasing trend.Compared with SEB, the surface microhardness of the sample after SEBSA is higher, and the surface hardness after W alloying is slightly higher than that after Mo alloying.
(4) After SEB and SEBSA, the surface was greatly improved.Compared with the wear loss of the matrix, the wear resistance after SEBSA increased by nearly six times.

Figure 2 .
Figure 2. Comparison and Analysis of Surface Hardness.

Figure 4 .
Figure 4. Microhardness of 45 steel section under different treatments.

Figure 5 .
Figure 5. Friction and wear amount of 45 steel samples under different treatments.