Surface Hardening: Status Quo and Trends

Surface hardening is a vital process in mold manufacturing. It enhances mold durability by increasing surface hardness, wear resistance, and fatigue strength.

There are common techniques, including plasma and Laser Nitriding, Laser Surface Hardening, TRD (thermal reactive deposition), and PVD (physical vapor deposition). Each offers specific advantages based on the project’s requirements, complexity, and budget. Recent surface hardening techniques for injection mold steels focus on improving wear resistance, hardness, and durability while minimizing distortion.

You can also read: How to Increase Injection Mold Life

Here are some of the most advanced methods currently in use:

Nitriding Processes

Nitriding, including plasma/ion and gas nitriding, remains among the most effective surface hardening methods for injection mold steels. It creates a hard, wear-resistant nitride layer with minimal distortion. Plasma nitriding, for instance, is advantageous for intricate molds because it allows selective hardening through mechanical masking, especially for parts with threaded holes. This method also maintains dimensional stability and delivers high corrosion resistance due to its dense compound zone of nitrides, it is suitable for molds that encounter both abrasive wear and corrosion.

Laser Surface Hardening (LSH):

Laser surface hardening has gained traction as a precise, non-contact method for surface hardening steels. It uses controlled laser heating to form a martensitic layer on the steel’s surface. It increases significantly its hardness without affecting the core properties. Advances in laser technology, including fiber and diode lasers, have made LSH a versatile option for molds requiring high precision and tailored hardness depths. This method has also incorporated modeling and simulation tools, such as finite element analysis and machine learning, to optimize parameters like laser power and scanning speed, enhancing process’s durability and efficiency

Laser Nitriding

A variation on traditional nitriding, laser nitriding combines laser energy with nitrogen to create a hardened, nitrided surface layer with improved wear and thermal resistance. This process is particularly beneficial for mold steels subjected to high temperatures and requires a hardened surface layer that maintains properties under cyclic thermal stress. Studies have shown that laser nitriding can achieve significant increases in surface hardness and wear resistance, beneficial for high-performance applications

​TRD and PVD

Plasma nitriding, for example, creates a hardened surface through nitrogen ion diffusion, enhancing resistance to wear and corrosion without compromising the alloy’s inherent toughness. Research conducted by an Asian university demonstrated that adjusting nitriding temperature improves both wear resistance and anti-friction properties, significantly extending mold lifespan.

The choice of surface hardening also depends on the application. For example, TRD is ideal for components exposed to high abrasion, while PVD suits molds requiring a precise, corrosion-resistant coating. Each treatment method targets key mold attributes, from hardness and surface quality to wear and corrosion resistance. Manufacturers can thus select the optimal treatment to extend the mold’s operational life and reduce maintenance costs.

Ongoing Developments: Laminar Plasma Jet (LPJ):

The laminar plasma jet (LPJ) is a novel heat source characterized by high thermal efficiency, process efficiency, and relatively low cost compared to lasers. LPJ generates a high heat flux by ionizing working gas via continuous arc discharge, enabling rapid surface heating. It shares the exact basic austenite-martensite transformation mechanism as laser surface hardening but relies on thermal conduction from the plasma, rather than optical-to-thermal energy conversion.

Advantages of LPJ:

• Lower Cost: LPJ is more economical than laser systems, making it accessible for broader industrial applications.
• Higher Efficiency: It offers faster processing times and better energy utilization.
• Localized Hardening: Like LSH, LPJ enables targeted surface modification, crucial for molds where defects often occur in specific areas.
• Improved Mechanical Properties: LPJ effectively enhances surface hardness, wear resistance, and corrosion resistance, extending the service life of treated components.

Quantitative Study Findings From Research

1. Microstructural Transformation: LPJ surface hardening transforms P20 mold steel’s microstructure from tempered martensite to compact lath-martensite. This transformation increases surface hardness from 300 to 600 HV and enhances residual compressive stress from -255.01 MPa to -752.34 MPa.
2. Wear Resistance: The coefficient of friction (COF) decreased from 0.314 to 0.186, while wear rates were reduced significantly. The wear mechanism shifted from abrasive and adhesive wear to mild oxidative wear due to the hardened microstructure.
3. Corrosion Resistance: LPJ treatment reduced the corrosion rate from 0.2427 mm/year to 0.0680 mm/year, primarily due to the formation of a dense lath-martensite structure and the homogenization of Cr distribution.

Final Considerations

Surface hardening is an investment in mold longevity and reliability. The right technique preserves mold integrity, minimizes downtime for repairs, and ensures consistent quality in mass production. By carefully selecting surface treatments, mold makers can optimize their molds for durability and performance, resulting in significant cost savings. For a complete perspective on mold-making success you can also read Mold Manufacturing: How to Choose the Best Alloy and
How Thermal Conductivity Drives Mold Efficiency.By aligning these, manufacturers can achieve optimal mold performance and cost-effectiveness. Again, check the specification and customer requirements before the definition of the manufacturing chain. 

To read more: Laminar plasma jet surface hardening of P20 mold steel: Analysis on the wear and corrosion behaviors.