Heat-Treatment Importance in H13 Tool Steel Microstructure Analysis for Texturing in Injection Molding

The automotive industry relies heavily on injection molding to produce a large quantity of plastic components, ranging from interior trim to structural elements. Selecting the right tool steel for these molds is crucial for ensuring part quality, production efficiency, and cost-effectiveness. Given the comprehensive requirements of plastic molds, surface finishing is of particular interest since the injected plastic part can replicate any detail (as well as defects) from the mold surface. Therefore, several aspects of mold finishing are relevant, especially those related to manufacturing conditions—machining, grinding, polishing, and texturing—as well as the quality of the tool steel, concerning microstructure homogeneity and the absence of non-metallic inclusions (cleanliness) [2]. Texturing is an engineering technology that can be used to enable surface functionalization in the plastics injection molding industry [3]. A variety of techniques are available for texturing tool steels. The most common mechanical methods include machining, grinding, and sandblasting. These methods can be used to create a wide range of textures, from fine lines to deep grooves. However, they can also be time-consuming and expensive. Chemical methods, such as etching and electroplating, offer another approach to texturing tool steels. These methods are often more precise than mechanical ones but may be limited in the types of textures that they can produce. Electrical methods, including laser texturing and electrical discharge machining (EDM), provide the highest precision and versatility for texturing tool steels. However, they are also the most costly. The optimal technique for texturing a tool steel depends on the desired texture, the tool's size and shape, and the budget. One of the most competitive and widely used texturing technologies is the chemical etching (ECT), where material removal occurs due to chemical reactions between reactive agents and the substrate (a microscopic electrochemical cell action similar to corrosion or chemical dissolution of metal). This process alters the surface topography without changing the substrate's mechanical properties [4]. Chemical agents used for texturing tool steels fall into two main categories, depending on the desired texture and application: Etchants, which are commonly used to reveal grain boundaries and microstructure details, are particularly effective for creating contrast between different phases [5, 6]. Surface texturing agents, which create a matte, slightly roughened surface suitable for improved lubrication and adhesion and for removing light oxide layers [7, 8]. When problems arise with textured steels, such as unexpected anisotropy in mechanical properties or premature failure, metallography can identify the underlying microstructural cause, facilitating corrective actions or process adjustments. Choosing the appropriate tool steel for injection molds is critical for ensuring successful production and a cost-effective operation. Different tool steels offer varying advantages and are suited for specific applications based on factors like production volume, the material for being molded, and the required surface finish. A diverse range of tool steels is used for varying automotive applications. Pre-hardened grades like P-20 offer cost-effective solutions for low-stress parts, while advanced grades like H13 and NAK80 provide superior strength and performance for demanding components. New developments in coatings and surface treatments further enhance wear and corrosion resistance. H13 steel is a versatile chromium-molybdenum hot-work steel, is widely used in hot and cold work tooling applications and is typically heat-treated to a hardness of about 48-52 HRC. Microscopy examination and metallography preparation of tool steels present challenges due to their high hardness, elevated carbide concentration, and specific microstructure. This intricate understanding of a metal's microstructure is invaluable in determining its mechanical properties, corrosion resistance, and overall performance. Metallography plays a crucial role in understanding and optimizing the properties of textured steels, revealing microstructural features that allow for detailed observation of grain size, shape, orientation, and the presence of primary and secondary phases. This information is vital for understanding the factors that influence the texture and its impact on the material's properties. Metallography helps establish the link between processing parameters and the resulting texture and properties by correlating the observed microstructure with the processing history. This knowledge is crucial for optimizing processing routes to achieve desired material characteristics. Here, two samples of as-received and heat-treated H13 tool steels are shown to highlight heat treatment effects in the final microstructure and properties. Nital solution 2% concentration (2 ml of HNO3 and 98 ml of Ethanol at 95%) was used at room temperature as etchants to reveal the most characteristic metallography phases. The microstructure was characterized by using a 20X-5000X magnification 970F Keyence optical microscope. A carefully controlled heat treatment procedure must be followed to achieve a specific hardness of 46 HRC in a commercial tool steel H13. This procedure includes stages of austenitization, quenching, and tempering. It is pivotal to acknowledge that specific conditions may vary depending on the material's exact composition, the component's dimensions, and the available heat treatment equipment. Austenitization temperature is typically around 1020°C to 1050°C. This stage is crucial for dissolving carbides in the matrix and forming austenite. The usual quenching media for H13 include forced air, oil, or salt baths. Quenching oils for tool steels can be categorized into different types based on their cooling speed, from fast quenching to slower cooling oils. A medium to fast cooling oil is generally preferred for commercial tool steel H13, depending on the cross-sectional area of the component and the details of the heat treatment process. After quenching, it is fundamental to carry on one or more tempering cycles to achieve the desired hardness of 46 HRC and enhance the material's toughness. Tempering is typically carried out at temperatures ranging from 550°C to 650°C (1020°F to 1200°F), depending on the desired final hardness and mechanical properties. Figure 1 shows the typical optical images showing the microstructures of heat-treated H13 tool-grade steel. The initial microstructure of the as-received sample of tool steel H13 in annealed condition consists of a ferritic matrix. The right image clearly shows the carbides distribution homogeneously, which contributes to relatively weak precipitation strengthening. As a result, after the solution treatment and subsequent oil cooling, the sample has an entirely martensitic structure. Optical micrographs of tool steel H13 heat-treated with a magnification of 500x (left) and 3000x (right).