Steel Hardenability and Failure Analysis

Brett A. Miller, Stork Technimet, Inc.


From: B.A. Miller, Steel Hardenability and Failure Analysis, Practical Failure Analysis, Vol 1 (No. 3), Jun 2001, p 45–50

Abstract: Hardenability evaluation is typically applied to heat treatment process control, but can also augment standard metallurgical failure analysis techniques for steel components. A comprehensive understanding of steel hardenability is an essential complement to the skills of the metallurgical failure analyst. The empirical information supplied by hardenability analysis can provide additional processing and service insight to the investigator. The intent of this paper is to describe some applications of steel thermal response concepts in failure analysis, and several case studies are included to illustrate these applications.

Keywords: Thermal response

Materials: ASTM A232 (Chromium-vanadium alloy steel), UNS K15047; ASTM A572 grade 50 (High-strength low-alloy (HSLA) steel), UNS K02303; 1045 (Nonresulfurized carbon steel), UNS G10450, UNS K02599; ASTM A36 (Nonresulfurized carbon steel), UNS G10450, UNS K02599

Failure types: Brittle fracture; Joining-related failures


The Basics of Hardenability

The primary characteristic of steel that allows property modification through heat treatment is hardenability. Hardenability is a term used to quantify the relative response of a material to thermal treatment transformation to martensite based upon alloying elements and other variables. Many researchers have empirically determined the individual and combined effects of alloying elements. A comparison measure, identified as the critical diameter(DI), was implemented to estimate the hardening response of a steel from the chemical composition. The critical diameter calculated for a given steel composition can thereafter be used to create an approximate through-thickness distance hardness profile. The individual alloying element contributions and complicated regression equations typically used are included in ASTM A 255 and SAE J406.

Alloying Effects on Hardenability

It has been demonstrated that almost all elements have a measurable effect on steel hardenability. Carbon has a profound effect on both the martensite hardness and the steel hardenability. The mathematical estimation for steel surface hardness is based upon carbon content alone. The predominant additional elements of interest in hardenability calculations are manganese, silicon, nickel, chromium, molybdenum, copper, vanadium, and boron. The decrease in hardness as a function of depth depends primarily upon the combined effects of these alloying elements. Most other common steel alloying elements have a minimal effect on hardenability. Generally, hardenability is directly proportional to elemental concentration, that is, higher amounts of an alloying element result in higher hardenability; however, a few elements exhibit an inverse proportionality.

Boron Steel Hardenability

Heat treatment problems and service failures in boron-containing steels can result from unanticipated hardenability variations. Boron has an effect on hardenability only second to carbon, although its efficacy is due to a different mechanism. In fully deoxidized steels of low carbon content, boron has been shown to increase hardening depth by various effects, including the suppression of the formation of both bainite and ferrite. Boron steels typically contain between 0.0005 and 0.0030 wt.% boron in the absence of high levels of other alloying elements.
Unlike other elements, the relative amount of boron beyond a requisite level will not provide additional increases in hardenability. Due to the reactivity of boron, this element can be rendered ineffective if the nitrogen in the steel is not completely combined by the addition of titanium or aluminum. Altered hardenability calculations for boron steels have been developed for estimation purposes.

Grain Size Effects

Grain size is another essential variable in steel hardenability calculations. Grain size and hardenability exhibit an inverse proportionality. Steels with lower grain size numbers, which correspond with large or coarse grains, exhibit superior hardening characteristics. The superior hardenability of large grained steels results because the number of pearlite nuclei is proportional to the available grain boundary area. Larger grained materials shift the nose of the Time-Temperature-Transformation (TTT) curve to the right, inhibiting pearlite nucleation and promoting martensite formation. Much of the hardenability research has been performed on a moderately fine grain size, Size 7 per ASTM E112, with correction factors determined for finer- and coarser-grained steels.

Variation and Inhomogeneity Effects

Complete compositional homogeneity is not achievable in engineering alloys, notwithstanding carefully worded marketing claims to the contrary. All alloy specifications include permissible ranges of individual elemental concentrations, which are necessary to keep these materials economically feasible. When hardenability is of paramount importance, ‘H’ grades of steels are frequently available. These steels specify a minimum hardenability as an independent variable and the composition as a dependent variable. Clearly, controls such as material and purchasing specifications can be successfully implemented to obtain steel that will yield predictable and suitable post-processing properties for component manufacture. In commercial failure analysis, however, any possible combination of grain sizes and chemical compositions, even extreme combinations, may be encountered and must be kept in consideration for their hardenability effects.
Trace levels of unspecified elements can also be high enough to impart undesired hardening response. For instance, many carbon steel specifications contain no maximum requirements for chromium, nickel, aluminum, and molybdenum, which can result in significant hardening at relatively low elemental concentrations. Compositional variations at different regions in a material can result in hardenability variations that may initiate failures. Chemical segregation or banding, carburization, and decarburization are a few of the types of elemental variations that can affect localized thermal response.
Additionally, grain size differentials can provide unpredicted hardening. Wide ranges of grain sizes can be produced in casting, forging, and other high temperature processes. Subsequent heat treatment of cold worked steels can result in recrystallization to finer grains with reduced hardenability. Recrystallization and grain growth of steel can result in significantly higher hardenability due to the larger grain size.

Hardenability Calculations

The mechanics of critical diameter calculation are fairly straightforward and have been extensively, but not exhaustively, documented. The calculation methodology is suitably described in SAE and ASTM specifications and is beyond the scope of this article. Many researchers have developed more specific methods for hardenability calculation, but these are less broadly applicable and are of more importance to the development of manufacturing processes than on the performance of failure analyses.
Widely published hardenability information may be too esoteric for easy interpretation by many engineers. However, the impact of hardenability on steel materials must be recognized. For illustration of the implications of alloying variation, the moderate hardenability Grade 4130 steel is selected, due to its widespread use in many industries. The alloying elements for this grade are specified as acceptance ranges. The calculated hardenability of this grade at maximum, midrange, and minimum elemental concentrations is illustrated in Fig. 1 for a steel with a grain size of 7. These compositional extremes correspond to calculated DI values between 38.1 and 99.1 mm (1.5 and 3.9 in.).
Figure 1

Fig. 1  The distance hardness is depicted for Grade 4130 steel at maximum, midrange, and minimum composition levels. This comparison is for Grain Size 7 only.

Consideration of the potential hardenability variations that can occur due to grain size differences reveals an even wider spectrum of possible critical diameter values. This is shown graphically in Fig. 2. The critical diameter range is between 33.0 and 116.8 mm (1.3 and 4.6 in.). To really understand the importance of this variation, it is necessary to diagram the distance hardness with the additional influence of grain size, as shown in Fig. 3. Addition of the approximate equivalent tensile strengths to this figure emphasizes that variable hardenability can result in drastic strength differences. Differences of this magnitude are not a trivial occurrence, as failure analysts often encounter such cases.
Figure 2

Fig. 2  A graphical representation of the spectrum of possible critical diameters that can be calculated for extreme compositional and grain size variations in Grade 4130 steel

Figure 3

Fig. 3  The distance hardness and approximate tensile strength equivalents that can result from composition and grain size extremes

Heat Treating and Hardenability

Quenching and tempering, and some surface hardening methods, are designed to take advantage of the beneficial aspects of steel hardenability. Carburizing, carbonitriding, and other methods use altered surface compositions to increase surface hardenability in applications when lower subsurface properties are desired. In many cases, failures can result directly from unanticipated heat treating results. It should be mentioned that heat treatment anomalies can also be due to austenitizing temperature, quench rate, quench delay, and many factors aside from chemical composition and grain size.

Welding and Hardenability

The hardenability variations described herein are also of great concern in the fusion welding of steels. The composition dictates the maximum heat affected zone (HAZ) and weld hardnesses along with a wide variety of heat input and cooling rate variables. The normal dilution effects from the use of differing base metals or differing filler metal cause graduated hardenability coupled with cooling rate variations. Very often, elemental concentrations are used along with geometric variables and restraint levels to estimate welding preheat temperatures in order to avoid high HAZ hardening.

Case History #1 - Compression Spring Fracture

A client submitted a helical compression spring that had failed during installation. The material was identified as rectangular chromium-vanadium steel wire per ASTM A 232. It was opined that the failure had occurred via hydrogen embrittlement during a nickel plating process after quenching, tempering, and coiling. The spring had a lower than average calculated DI of 96.5 mm (3.8 in.) with a grain size of 7, but it had an elevated surface hardness because the carbon content was at the high-end of the allowable range. The nominal critical diameter for this steel is approximately 101.6 mm (4.0 in.). Laboratory analyses showed the failure occurred by quench cracking rather than hydrogen embrittlement.
A metallographic cross section at the likely origin of a quench crack on the spring is shown in Fig. 4. The profile of the crack appears intergranular, which was consistent with fractographic features observed during scanning electron microscope examination. A plating layer is evident over a portion of the crack surface, confirming that cracking occurred prior to the plating process. The heat treatment process was likely developed for steel heats with more nominal carbon content. These lower carbon heats would require a more severe quenching practice to satisfy the high surface hardness requirements.
Figure 4

Fig. 4  Metallographic cross section through a fractured compression spring. Intergranular features are suggested and a plating layer is apparent on the quench crack surface. 2% Nital Etchant (Original Magnification 100 ×)

Case History #2 - Axle Bar Cracking

Axle bars were manufactured from 38 mm square Grade 1045 plain carbon steel. The bars were induction hardened to 45 to 50 HRC with a minimum required case depth of 3 mm. The bars were cracking during installation but no magnetic particle indications were identified prior to installation. Laboratory investigation revealed intergranular, brittle fracture morphology followed by transgranular cleavage within the core. Hardenability evaluation revealed a DI of 45.7 mm (1.8 in.) due to the presence of trace elements. The typical DI of Grade 1045 steel is 22.9 mm (0.9 in.). A high carbon content (0.52%) was also identified, further contributing to high core hardness. Hardenability evaluation and tempering curve inspection revealed that the tempering temperature necessary to reduce the material to the specified hardness at the specified depth was likely within the blue brittleness range of 230 to 370 °C (450 to 750 °F). This embrittlement phenomenon was considered the likely cause of the failures.

Case History #3 - HSLA Steel Welding Failure

In-service cracking occurred to a large piece of mining equipment. The vehicle was fabricated from ASTM A 572 Grade 50 High Strength Low Alloy (HSLA) steel, ranging in thickness from 38 to 152 mm. Fatigue fracture was found to have initiated in a weld HAZ in 127 mm thick plate. The plate met the grade specifications and calculation revealed a DI of approximately 38.1 mm (1.5 in.). The equivalent HAZ hardness was found to be 45 HRC, exactly as predicted by hardenability evaluation. It was determined that the welding procedure omitted a preheating step, resulting in extremely rapid solidification and cooling of the weld, equating to an effective “quench” in normal heat treatment practice. In addition to poor fatigue strength in the resulting weld joints, some hot cracking was apparent, but not at the fracture origin.

Case History #4 - Weld Filler Metal Cracking

A carbon steel actuator that exhibited brittle cracking within a weld was submitted for metallographic examination. The base materials were identified as ASTM A 36 structural steel and the joint was a corner fillet. The weld cross section is shown in Fig. 5. The weld quality appeared good, but anomalous etching behavior, and subsequent chemical analysis, revealed that the weld filler was an austenitic stainless steel. The weld joint design resulted in extreme dilution of the filler metal, due to the relatively small amount of filler used. As a result, a dilution zone with insufficient nickel to remain austenitic at room temperature was formed. This zone also contained sufficient chromium, silicon, and molybdenum from the filler metal, and carbon from the base metal, to approximate a highly hardenable low alloy steel upon fast solidification and cooling. The base metal had a DI of 10.2 mm (0.4 in.), and the average weld DI was likely greater than 68.6 mm (2.7 in.). The measured equivalent hardnesses were 74 HRB in the base metal, 89 HRB in the HAZ, and 46 HRC in the weld. The stainless steel filler metal was found to have been an inadvertent, costly, and disastrous substitution.
Figure 5

Fig. 5  Metallographic cross section through a carbon steel component inadvertently welded with an austenitic stainless steel filler metal. Brittle cracking occurred in a very hard region in the center of the weld. 2% Nital Etchant (Original Magnification 12.5 ×)

Other Heating Event Failures

In addition to intentional heat treatment and welding effects, there are a variety of other processes that may result in hardening-related failures. Thermal events in manufacturing, which are not intended to result in hardening, such as casting, forging, annealing, and normalizing, can all result in severe property differences, depending upon alloying and cooling rates. Cutting techniques with insufficient cooling can result in hard spots. Hardened zones can also result from abusive grinding, localized wear, or inadvertent frictional heating of moving components. Unanticipated and unusual events such as resistance heating or electrical arcing of poorly grounded equipment may result in localized hardening and subsequent failure.

Summary

An understanding of the significant effects of chemical composition on the heat treatment response and mechanical properties of steels is necessary for the practicing metallurgical failure analyst. The allowable and discrepant alloying variations present in engineering steels can result in drastically different properties after heat treatment or welding. Grain size variation can also significantly change the hardenability of materials ostensibly of the same grade.
Failures occurring due to excessive hardenability have been emphasized, as this can often lead to brittle, catastrophic failures, whereas low hardenability more likely may result in deformation or ductile overload failure. Fatigue failures are somewhat problematic with regard to hardenability, as fatigue strength and mechanical strength do not always exhibit a direct proportionality. As with any tool, hardenability evaluation must be used with the requisite discretion of the failure analyst, as this information is most often illustrative rather than exact.

Related Information

Failures Related to Welding, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 156–191
W.T. Becker and D. McGarry, Mechanisms and Appearances of Ductile and Brittle Fracture in Metals, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 587–626