H13 steel is an important hot working mold steel, also known as 4Cr5MoSiV1. It has high hardenability, hardness, good toughness, thermal fatigue resistance, heat resistance, thermal stability, oxidation resistance and corrosion resistance. It is widely used in the manufacture of various molds [1
]. The development of the mold industry affects the manufacturing level of a country; despite the rapid development of China’s mold industry, the service life of mold steel is low, and the waste caused by the low service life of mold is about several hundred million yuan each year. Improving the life of the mold is a very urgent task and has become an increasingly prominent problem.
In the process of production and use, H13 hot work molds are constantly subjected to physical, chemical effects such as wear, thermal fatigue, erosion, stress corrosion, as well as the effects of cold and heat alternation and impact loads [3
]. In such a harsh environment, molds are prone to thermal fatigue, thermal wear, and cracking [5
]. As we all know, the failure of the mold starts most frequently on the surface [6
]. In other words, improving the surface microstructure and performance can effectively increase the service life of the mold. Therefore, the necessary surface strengthening measures for hot work molds are an important means to improve the service life of molds.
Compared with other surface modification technologies, laser cladding technology has advantages not available in other methods. Not only can it obtain a coating with an ideal thickness, but also the metallurgical combination of the coating and the substrate has a high bonding strength, which can effectively improve the wear resistance and oxidation and corrosion resistance of the tool surface and has great application and development prospects [7
]. In addition, the Stoney equation and some specific devices are proposed to be used in coatings characterization, which can detect and measure stresses on fine layers [8
]. Present and future works must use these new ideas. The development of these theories and methods has further promoted the application of laser cladding technology.
Laser cladding materials mainly include self-fluxing alloy materials, ceramic materials, and composite materials [9
]. Self-fluxing alloy refers to an alloy with iron, cobalt and nickel as the matrix, adding elements such as Si and B that have strong deoxidation and self-melting effects [10
]. Ceramic powder mainly includes oxide ceramic powder and carbide ceramic powder. Oxide ceramic powders mainly include Al2
, etc., which have good high temperature resistance, wear resistance, corrosion resistance and other properties. Carbide ceramic powders mainly include WC, TiC, SiC, Cr3
, etc., which have high hardness and good abrasion resistance [11
Due to development needs, people’s requirements for material properties are getting higher and higher. It is difficult for a single material coating to simultaneously meet the improvement of multiple aspects of the material, while composite materials can improve many properties of materials [12
]. In order to further improve the properties of the coating, ceramic particles are added to the self-fluxing alloy powder. However, the addition of ceramic particles makes the thermal expansion coefficient and elastic modulus of the composite coating significantly different from that of the substrate. During the cladding process, thermal stress is caused by the huge performance difference between the coating and the substrate, which makes the combination of the coating and the substrate poor [14
Therefore, gradient materials have become a key to solve the problem. In 1987, the Japanese Science and Technology Agency proposed “gradient functional materials” [15
]. The design idea is to make the physical and chemical properties of coating gradually and continuously transition to avoid the abrupt change of performance from the substrate to the coating [16
], alleviate the stress condition at the interface, improve the bonding condition between the coating interfaces so that the coating can obtain appropriate hardness, toughness and residual stress, so as to effectively inhibit the generation and expansion of cracks [17
], improve the load-bearing capacity and anti-friction properties of the coating.
In order to improve the service life of H13 hot work steel, cobalt-based coatings with a gradient effect were prepared on the surface of the H13 steel substrate, and the performance of the coatings was measured by the experiments.
2. Experimental Materials and Methods
2.1. Material and Sample Preparation
H13 steel was supplied by the supplier and has been quenched and tempered, and the microstructure is tempered sorbite. The test piece was analyzed by EDS(Oxford Instruments, Shanghai, China), and its chemical composition is shown in Table 1
Molybdenum, chromium and other elements improve the hardenability, fatigue resistance and oxidation resistance of H13 steel. Silicon improves the decarburization sensitivity of the steel. Vanadium strengthens the second hardening of the steel and improves the thermal stability of the steel [18
Because of its excellent wear resistance, corrosion resistance and high temperature resistance, the Co-based ceramic reinforced composite coating has become a hot research topic of laser cladding surface modification. In this test, different contents of WC ceramic particles were added to Co-based self-fluxing alloy powder. The chemical composition of Co-based alloy powder is shown in Table 2
, and the data was provided by the supplier. The purity of WC is 99.9%, and the particle size is 2.5–3.5 μm.
Before the laser cladding test, the surfaces of the substrates need to be conditioned. The surfaces of the substrates were polished from coarse to fine with different types of sandpaper to remove the surface oxide layer and oil stains. The polished samples were ultrasonically cleaned with absolute ethanol and put into a drying box for drying treatment.
The cladding powder used in this test was prepared by electronic balance. WC powder was added to the Co-based alloy powder at a mass fraction of 5%, 10%, 15%, 20%, and 25%. The prepared composite powders were separately placed into a planetary ball mill. The ball-to-material ratio was 10:1, and the ball milling time was 6 h, which made the composite powder mix evenly and refine the particles. Finally, the mixed powders were put into a drying box and dried at 100 °C.
The H13 steel substrates were preheated at a temperature of 200 °C to reduce the thermal stress caused by the excessive temperature difference between the substrate and the coating during cladding, thereby reducing cracks and ensuring the quality of the coating.
In this test, three samples with Co-based gradient coatings were prepared, each sample has three cladding layers, and the mass fraction of WC increases from bottom to top. The proportion of gradient coating composite powder is shown in Table 3
In the process of laser cladding, several main parameters, such as defocusing amount, frequency, scanning speed, pulse width, single pulse energy and so on, were mainly considered to affect the performance of the cladding layer, and the optimal combination parameters were finally determined, as shown in Table 4
2.2. Analysis Methods
The samples were cut into small pieces of 10 × 10 × 10 mm, sanded with 200–1500 abrasive paper, and then polished with diamond polishing agent. The cross section of the gradient coating was viewed with a scanning electron microscope (HITACHI, Tokyo, Japan) to evaluate the quality of the cladding.
Then, the samples were etched with aqua regia (HCl:HNO3 = 3:1). During the corrosion process, attention should be paid to the corrosion time. The microstructure of the gradient coatings were observed with a metallographic microscope (Shang Guang, Shanghai, China).
The 402MVD micro Vickers hardness tester was used to test the microhardness of the samples. The load was 100 g, and the loading time was 15 s. The hardness of the gradient coating section was measured every 0.2 mm from the coating surface to the substrate. Five points were measured at different positions of the same horizontal line each time, and the average value was taken.
The reciprocating friction and wear test was carried out with RTEC MFT-50 friction and wear tester. (Rtec, Nanjing, Jiangsu, China) The grinding specimen was a silicon nitride ceramic ball. The parameters set in the test were: test force 50 N, reciprocating stroke 10 mm, reciprocating frequency 30 Hz, wear time 1800 s, test temperature 17 °C humidity 50%. The weight loss of the samples was measured by an analytical balance with an accuracy of 0.0001 g, and the wear morphology was observed with a scanning electron microscope.
The impact test was carried out according to GB/T229-2007 “Metal Charpy Notched Impact Test Method”. The size of the impact specimens is 10 × 10 × 55 mm. The V-notch is processed in the middle of the impact surface of the substrate, the angle is 45°, the depth is 2 mm, and the radius of curvature at the bottom is 0.25 mm. The impact test equipment was a JBN-300C microcomputer-controlled pendulum impact test machine (Ji’nan Shijin Group Co. Ltd., Jinan, Shandong, China), and the test temperature was 17 °C. The schematic diagram of the impact test is shown in Figure 1
The static corrosion method was used to test the corrosion performance of the samples. The four sides and the bottom of the samples were sealed with RTV silicone rubber, only the coating to be corroded was exposed. Then, the samples were put into the self-made corrosion solution, the ratio of nitric acid and hydrochloric acid was 1:1, and the concentration was 15%. The different samples were respectively immersed in the corrosion solution for 24 h, and then they were taken out and put into anhydrous ethanol for ultrasonic cleaning and drying. The weight loss before and after the corrosion test was compared, and the morphology after corrosion was observed.
In this study, the Co-based gradient coatings were prepared on the surface of H13 die steel, and the performance differences between the base material and gradient coatings were analyzed by means of microstructure observation, hardness measurement, friction and wear test, impact fracture test, corrosion test, surface and section morphology observation.
The gradient coating forms a good metallurgical bond with the substrate. From the substrate to the coating surface, the hard phase content gradually increases and the microstructure gradually refines. The hardness and wear resistance of the coatings increase with the increase of the hard phase content, the strengthening mechanism is fine grain strengthening, solid solution strengthening and dispersion strengthening.
The main wear mechanism of the substrate surface is ploughing, and the main wear mechanism of the gradient coating surface is brittle peeling. The higher the content of the hard phase, the more obvious the brittle peeling phenomenon.
The impact resistance of the gradient coating is lower than that of the substrate, and the brittleness of the coating increases with the increase of hardness. The fracture mode of the substrate is a ductile fracture, and the fracture mode of the coating is a brittle fracture.
The gradient coatings effectively improve the corrosion resistance of the base material, and the higher the hard phase content, the better the coating’s corrosion resistance.