George Vander Voort has a background in physical, process and mechanical metallurgy and has been performing metallographic studies for nearly 50 years. He is a long-time member of ASTM Committee E-4 on metallography and has published extensively in metallography and failure analysis. He regularly teaches MEI courses for ASM International and is now doing webinars. He is a consultant for Struers Inc. and will be teaching courses soon for them. His website, www.georgevandervoort.com, not only details his consulting services but also houses over one hundred articles, studies, or instructional graphics on topics related to physical, process and mechanical metallurgy. The following is an overview and an excerpt of failure factors from “Identifying the Cause of Tool and Die Failure”, published in 2016. There are particular elements to this study which relate to the heat treat industry.
Steels used for tools and dies differ from most other steels in several aspects. First, they are used in the manufacture of other products by a variety of forming processes. Second, tools and dies are generally used at a higher hardness than most other steel products; 58 to 68 Rockwell C is a typical range. Dies for plastic molding or hot working are usually used at a lower hardness, typically from 30 to 55 Rockwell C.
These high hardness values are required to resist anticipated service stresses and to provide wear resistance. However, the steels must also be tough enough to accommodate service stresses and strains without cracking. Premature failure caused by cracking must be avoided, or at least minimized, to maintain minimum manufacturing costs. Unexpected tool and die failure can shut down a manufacturing line and disrupt production scheduling. Tools and dies must also be produced with the proper size and shape after hardening so that excessive finishing work is not required. Heat-treatment distortion must be controlled, and surface chemistries must not be altered. Because of the careful balance that must be maintained in heat treatment, control of the heat-treatment process is one of the most critical steps in producing successful tools and dies. In addition to controlling the heat-treatment process, tool and die design and steel selection are integral factors in achieving tool and die integrity.
The following list is excerpted and abridged to highlight phases or processes related to heat-treat. The explanation behind each factor is available at the original post.
A number of factors can be responsible for tool and die failures. They include:
1. Mechanical design. The design must be compatible with the steel grade selected, the procedures required to manufacture the tool or die, and the use of the tool or die. . . .
The importance of good design cannot be overemphasized. Poor design can cause or promote heat-treatment failures before any service life is obtained, or it may reduce service life dramatically.
In designing a tool or die, a host of factors must be considered. In practice, separating the design stage from grade selection is difficult because the two steps are interdependent. The choice of a certain grade of steel, such as one that must be brine- or water-quenched, will have a substantial bearing on all aspects of design and manufacture. In general, any steel grade that requires liquid quenching demands very conservative, careful design.
Air-hardening grades tolerate some design and manufacturing considerations that could never be endured by a liquid quenching grade. The design must also be compatible with the equipment available–heat-treatment furnaces and surface-finishing devices, for example. . . .
2. Grade selection. The grade of steel selected must be compatible with the design chosen, the manufacturing processes used to produce the tool or die, and the intended service conditions and desired life. . . .
3. Steel quality. The material must be macrostructurally sound, free of harmful inclusions to the degree required for the application, and free of harmful surface defects.
Despite the care taken in the manufacture and inspection of tool steels, faulty materials occasionally cause tool and die failures. However, such problems are rare. The most common of these defects are voids from secondary pipe, hydrogen flakes, surface cracks, porosity or microvoids, cooling cracks, segregation, and poor carbide distributions. Improper control of annealing may also produce non-uniform carbide distribution or carbide networks that may influence heat-treatment uniformity, lower ductility, or impair machinability.
4. Machining processes. The machining processes used to produce the tool or die must not alter the surface microstructure or surface finish and must not produce excessive residual stresses that will promote heat-treatment problems or service failures.
Machining problems are a common cause of tool and die failures. It is generally best to avoid machining directly to the finish size unless pre-hardened die steels are used. Obtaining perfect control of surface chemistry and size during heat treatment is difficult. Thus, some final grinding is usually needed after heat treatment. The presence of decarburization is generally quite detrimental. Also, because stresses are high in heat treatment and in service, rough machining marks must be avoided. Identification stamp marks are another common source of failures in heat treatment and in service; they should be avoided.
5. Heat-treatment operation. Heat treatment of tools and dies must produce the desired microstructure, hardness, toughness, and hardness at the surface and in the interior.
Improper heat-treatment procedures are the single largest source of failures during heat treatment, in subsequent processing steps, or in service. Each tool steel grade has a recommended austenitizing temperature range, which is generally rather narrow; a recommended quench medium; and recommended tempering temperatures and times for optimum properties. Some grades are more forgiving than others.
6. Grinding and finishing operations. Grinding and finishing operations must not impair the surface integrity of the component.
7. Tool and die setup. Alignment of tools and dies must be precise to prevent irregular, excessive stresses that will accelerate wear or cause cracking.
8. Tool and die operation.
Read the full study and report, including images of tool steel failure examples, at “Identifying the Cause of Tool and Die Failure”.
