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R.R. Blackwood Tenaxol Inc., Milwaukee, WI USA - Diagram of hardening and tempering Unfortunately, when cracking is encountered, it is often attributed to the severity of the quenching medium without microstructural verification. Although excessive quench severity is often the cause of quench cracking, there are many other sources that must also be considered. This paper will provide a tutorial on microstructural identification of various sources of steel cracking during heat treatment including: quench severity, prior steel structures, transformation temperature range, A. Quench Cracking Related to Severity of One major source of cracking is excessive cooling rates during the quench (quench severity). This is illustrated in Figure 2. Note that the crack passes straight from the Excessive cooling rates (high quench severity) will produce great er thermal stresses in addition to greater transformation stresses. (Steel cracking during transformation to martensite is Micrograph of AISI 4340 quenched and tempered steel illustrating macroetched pure Quench Cracking Related to Non- It is important to note that quenchant- related problems, other than excessive quench severity, are also often major contributors to steel cracking. These include: non-uniform fluid flow around the part due to poor racking of the parts prior to the quench, incompatible fluid contamination (oil -in-water or water-in-oil), - Micrograph of AISI 1045 steel as- quenched and tempered, microstructure shows bands with banded tempered martensite and some bainite. The crack profile revealed evidence of tempering oxide and secondary - Micrograph of AISI 1045 steel as quenched and tempered, illustrates representative under-heated microstructure observed adjacent on forged AISI 403 stainless steel valve stems that exhibited longitudinal cracking after quenching. Microstructural analysis suggested that cracking was caused by thermal stresses during forging or during heating prior to forging. Figure 5 illustrates evidence of a coarse grain condition associated with high-temperature surface oxidation. Further examination revealed evidence of high and low thermal oxidation within the crack profile . The presence of this condition suggests cracking occurred prior to, or during, forging. Figure 5 - Micrograph of AISI 403 stainless steel as-forged; microstructure is predominantly a mixture of carbide particles in a matrix of ferrite. No evidence of quenching and tempering was observed. High and low temperature oxidation can be observed on the surface of the sample and within the crack profile. due to the inherent weakness of the material Figure 6 - Micrograph of AISI 4140 steel as quenched and tempered; microstructure is tempered coarse grain martensite with intergranular quench cracking along the prior austenite grain boundaries.. (Magnification Figure 7 - Micrograph of AISI 4140 steel as quenched and tempered; microstructure is tempered martensite with evidence of temperature oxidation on the surface of the crack profile. Table 1 1065 1500 525 300 1090 1625 420 175 1335 1550 640 450 3140 1550 630 440 4130 1600 710 550 4140 1550 640 525 4340 1550 550 330 4640 1550 640 490 5140 1550 630 460 8630 1600 690 540 8695 1550 275 -- 9442 1575 620 410 Excessively high austenitizing temperatures increas temperature differentials which results in a corresponding increase in residual stress and G. Stress Risers from Prior Machining, Laps Surface conditions from prior machining conditions will act as stress risers which are areas of dimensional changes (Figure Figure 8 - Micrograph of AISI 4140 steel as quenched and tempered: microstructure is tempered martensite w ith quench crack in the area of dimensional change. (Magnification: - Micrograph of AISI 4142 steel as quenched and tempered; microstructure is Figure 10 - Micrograph of AISI 4140 steel as quenched and tempered; microstructure is tempered martensite with quench crack initiating from machine groove.(Magnification: 100X; Figure 11 - Micrograph of AISI 4118 carburized steel as quenched and tempered; microstructure Figure 12 - Micrograph of AISI 8630 steel as quenched; microstructure is martensite where cracking initiated from rolling seam. Figure 13 - Micrograph of AISI Type 403 stainless steel as quenched and tempered; microstructure is tempered; microstructure is predominantly tempered martensite with cracking promoted by the seam. (Magnification Figure 14 - Micrograph of AISI 1030 steel as direct forge-quenched and tempered; microstructure is tempered martensite (unetched) with forged in scale Figure 15 Micrograph of AISI 1045 as-forged steel illustrating a forging lap. (Magnification 30x, 2% Nital) Elemental analysis shored that the carbon content of the steel was higher than the specification value (3.56% C versus 3.10-3.45 % C). The surface hardness was 52HRC versus a specification value of 45HRC. The hardness was 94HRB. Microstructural evidence suggested quenching from an excessively high austenitizing temperature which contributed to Figure 16 Micrograph of induction hardened Figure 17 . Micrograph of induction hardened AISI G-3500 gray iron illustrating crack propagation into the induction hardened case. (Magnification 100X, 3% Nital) Another common source of steel cracking problems is alloy inclusions due to poor homogenization of the steel composition. The most common alloy inclusions are sulfides, silicates, oxides and scale. Examples are provided in Figures 18-22. In addition to microstructural analysis, elemental analysis may also provide an invaluable insight into potential for inclusion formation. For example, sulfide inclusions may be obtained if insufficient manganese is present. Generally, the manganese content should be approximately five times the sulfur content to convert sulfur to manganese sulfide. Figure 18 - Micrograph of AISI 4140 steel as quenched and tempered; micristructure is Figure 19 - Micrograph of AISI 4140 steel as quenched and tempered; microstructure is Figure 20 - Micrograph of AISI 1144 steel as quenched and tempered; microstructure is tempered martensite where cracking is promoted by inclusion defects. (Magnification 200X; Figure 21 - Micrograph of AISI 1144 steel as quenched and tempered; microstructure is Figure 22 - Micrograph of AISI 4150 steel as quenched and tempered; cracking initiates from silicate and sulfide inclusions. (Magnification: Figure 23 - Micrograph of AISI 4140 steel as quenched and tempered; microstructure shows bands of tempered martensite and tempered Figure 24 - Micrograph of AISI 4140 steel as quenched and tempered, microstructure shows bands with banded tempered martensite and tempered martensite/bainite microstructure illustrated in Figure 20. Here a representative view of subsurface cracki ng that was obtained is illustrated. (Magnificati In a similar case, AISI 1144, a resulfurized steel, pins were with subsequent cracking at the pin tips accompanied with soft spotting. (The pins were through-hardened prior to induction hardening of the pin tip.) Microstructural analysis showed that the cracking and soft spotting condition was due to stringer inclusions with bands of gross chemical segregation, significantly greater than normally observed with this grade of steel. The stringers act as stress concentration sites for crack initiation in the presence of quenching stresses. (See Figure 25) Figure 25 - Micrograph of AISI 1144 steel as quenched; microstructure shows shows ferrite bands and inclusions exiting the pin tip. Figure 26 - Micrograph of AISI 4140 steel as quenched and tempered; microstructure is tempered martensite where cracking is promoted Figure 27 - Micrograph of AISI 8630 cast steel as quenched and tempered; microstructure is tempered martensite, pearlite and ferrite showing a potential cracking condition. (Magnigication hardenability. Therefore, if the steel chemistry is incorrect, the selected quench process conditions may, if too severe, lead to cracking. Unfortunately, this problem is not uncommon. An example illustrating this problem was afforded by a quench cracking problem obtained with AISI 1070 st eel bearing raceways. Metalographic analysis confirmed that quench cracking had occurred. However, the steel chemistry, see Table 2, was incorrect for a plain AISI 1070 steel. The higher carbon, higher hardenability stee with a high manganese content, would be more susceptible to quench cracking using the same quenchant that the 1070 heat treating process demands. Table 2 AISI 1070 Steel Used for Bearing Raceways 1070 (%) (%) Carbon 0.65-0.75 0.74 Manganese 0.60-0.90 0.97 Phosphorous 0.11 0.04 Sulfur 0.026 0.05 Silicon 0.10-0.20 0.23 Nickel - 0.07 Chromium - 0.11 Molybdenum - 0.22 Copper - 0.10 Although quench cracking of steel may arise from insufficiently low quench severity, there are numerous other potential contributors to this problem. They include: non-uniform quenching due to poor system design, racking procedures which inhibit uniform quenchant flow around the part during the quench or contaminated quenchants. However, other potential sources of cracking are due to mechanical or material flaws which include: non-metallic inclusions, laps or seams, stress risers from prior machining, alloy non-uniformity and porosity.