|Fig. 1. In-service D2 tool-steel component failure due to quench cracking (Photograph courtesy of Aston Metallurgical Services Co., Inc.)|
It is important for heat treaters to understand the mechanisms associated with quench cracking and to take all prudent steps necessary to avoid in-service product failures (Fig. 1). To accomplish this task, we must work with design and manufacturing engineers on materials selection, manufacturing methods (including heat treatment) and safety allowances. One of the first steps in this process is to understand how flaws in materials initiate failures and how heat treatment might contribute to the problem. Let’s learn more.
|Fig. 2. Fracture path in D2 tool steel (Photograph courtesy of Aston Metallurgical Services Co., Inc.)|
The critical flaw size in a material is defined as the size of a flaw that will cause failure of the component at the expected operational stress level. Flaws exist in most engineered materials and may be characterized as cracks, voids, inclusions, weld defects or design/manufacturing discontinuities acting singularly or in combination with one another (Fig. 2). Simply stated, flaws are responsible for most parts failing.
Flaws are stress concentrators. It is also important to understand that surface cracks and internal cracks are not the same (large surface cracks being worst) and long, thin cracks are especially bad (since they have a lower radius of curvature and propagate under lower stress conditions). Any applied stress at the surface rises to a maximum value near the crack. In addition, applied loads will not distribute themselves over cracks. The size, orientation and distribution of cracks in a material influence which cracks will grow under stress and to what extent. Remember, once initiated, cracks propagate at the speed of sound.
There are three classic modes of fracture associated with cracks: tensile (Mode I), sliding (Mode II) and tearing (Mode III). Stress intensity (e.g., KI for Mode I) is a function of loading (i.e. applied stress), crack size and geometry. If KI represents the level of stress at the tip of a crack, the fracture toughness, KIC, is the highest value of stress intensity that a material can withstand (under very specific conditions) without fracture. Fast fracture occurs in a stressed material either when the crack reaches a critical size or a critical stress value. Perhaps surprisingly, the combination of critical stress and critical crack length at which fast fracture occurs is a material constant.
Ductile fractures are usually more desirable than brittle fractures since they normally provide some form of warning before failure, whereas brittle failures do not since there is little or no plastic deformation at strain rates typically under 5%. In general, temperature determines the amount of brittle or ductile fracture that can occur in a material. At higher temperatures, the yield strength is lowered and fracture tends to be more ductile in nature. On the opposite end (at lower temperatures) the yield strength is greater and fracture tends to be more brittle in nature. At moderate temperatures (with respect to the material), the material exhibits characteristics of both types of fracture.
Failure types depend on both temperature and stress. For noncyclic stress conditions and temperatures under 0.4 times the melting point, failure stress decreases with increasing maximum flaw size, rate of loading or decreased temperature. For temperatures greater than 0.4 times the melting point, time to failure decreases as stress or temperature increases.
Certain heat-treatment processes such as hardening and quenching tend to increase the internal stress state of a material. Improper heating to austenitizing temperature can result in thermally induced stress, which may cause a flaw to open up into a crack. As the material is heated, it undergoes volumetric changes due to phase transformation as well as thermal expansion. Rapid heating only accentuates this condition. Cracks occurring during heating are most often related to material imperfections, seams and inclusions.
Rapid or uneven cooling, especially when transforming the microstructure to martensite, also creates additional internal stresses. Holes, sharp edges, grooves, slots and corners can all be potential stress risers and crack-initiation zones. At a sharp edge or edge of a hole, for example, the heating and cooling rates can be substantially higher than the surrounding areas, putting tremendous strain on the material in these regions. While those features may be necessary in the component, it is important to exercise good engineering practices and properly chamfer or radius those areas to prevent sharp corners and edges. In induction heating, for example, certain materials may be placed in holes and other critical areas to help act as a heat sink and dampen the shock during the quenching operation. This can be costly, however, and the efficiency of the heat-treat operation may suffer.
The common ways in which quench cracking can occur from heat treating include:
- Improper steel selection: Selecting a steel with too high a hardenability can result in susceptibility to quench cracks and excessive core hardness.
- Improper part design: Sharp changes of section, lack of radii, holes, sharp keyways and unbalanced sectional mass create stress risers in locations where cracking is likely to occur.
- Inadequate stock removal: During original machining, remnants of seams or other surface imperfections act as a nucleation site for quench cracking during subsequent thermal operations.
- Overheating: Temperature overshoot during the austenitizing portion of the heat-treatment cycle can coarsen normally fine-grained steels. Coarse-grained steels increase hardening depth but are more prone to quench cracking than fine-grain steels. Overheating and excessively long dwell times should be avoided while austenitizing.
- Improper quenchant selection: Using an overly aggressive quenchant (e.g., water, brine or caustic) when a less severe quench media (e.g., polymer, oil) will work is a common cause of part cracking.
- Improper fixturing and entry of the part into the quenchant: A part should enter the quench medium with as little interference as possible. Differences in cooling rates can be created, for example, if parts are nested together in a basket, resulting in the parts along the edges cooling faster than those in the mass in the center. Part geometry can also interfere with quenchant delivery and effectiveness.
- Long lag times between quenching and tempering: Tempering parts as soon as practical will help avoid internal stresses from building up and being relieved by cracking. Certain high-hardenability materials, such as 4340, are particularly prone to quench cracking and must be tempered immediately (usually within 15 minutes of quenching) to avoid problems.
Material and process selection can contribute to cracking as well. For example, high-carbon and alloy steels with high hardenability often exhibit a greater tendency to initiate and propagate cracks. Even if they do not crack, materials that are heat treated to very high strength levels may contain localized concentrations of very high residual stress. If these stresses are aligned in the same direction as the applied load in service, catastrophic failure can occur. Care must also be taken to temper all materials before subsequent operations.
Cracking can also be encountered in materials such as iron, nickel and cobalt superalloys (the phenomena is called “fire cracking” or “strain-age cracking” or “stress cracking”) and occurs in both age-hardenable and solid-solution alloys. Causes for this condition have been traced to such items as high residual tensile stresses on the part surface, a strain-intolerance microstructure and the presence of stress risers. Steps to prevent cracking in superalloy materials include: reducing (or eliminating) residual surface tensile stresses (e.g., shot peening); modifying or redesigning part geometry (to eliminate stress risers); and addressing surface-roughness issues.
Avoiding premature failure of component parts, whether due to improper material selection, design, manufacture or heat treatment, is a goal shared by all. It is critical, therefore, that heat treaters understand enough fracture mechanics (the study of the propagation of cracks in materials) and engineers understand the effect of heat treatment so as to eliminate in-service performance issues. IH