A Case of Aluminum Nitride Embrittlement of Heavy Wall Cast Steel
Weld cracking was observed in multiple heats of large-sized cast steel components. Fracture analysis, metallography, and mechanical testing indicated that the failure mechanism was aluminum nitride (AlN) embrittlement. Prevention of this type of embrittlement is achieved by controlling aluminum content, nitrogen content, and above all, cooling rate of the casting. A macro-etch procedure for evaluating large castings for AlN embrittlement has been available in ASTM A 703 for decades, and it was found to adequately predict embrittlement in this instance.
KeywordsAluminum nitride embrittlement AlN embrittlement Cast steel Steel embrittlement Aluminum nitride ASTM A 703 ASTM A703
A manufacturer of heavy machinery reported weld cracking of cast modified AISI 8627 steel pressure-retaining castings during assembly. The cracking was discovered only after multiple failed leak tests and subsequent dye penetrant testing. Some of the samples obtained for laboratory testing had been unsuccessfully weld repaired over the original weld cracks.
Elemental analysis of the fracture surface platelets by energy-dispersive spectroscopy (EDS) could not reliably show increased aluminum concentrations relative to bulk analysis, and nitrogen is not detected with sufficient signal strength by the EDS method to be meaningful. Similarly, attempts at elemental mapping by EDS of the facets indicated no increased contrast due to aluminum in the platelet areas. This is typical of aluminum nitride embrittlement, because the AlN platelets or films are too thin to develop sufficient signal with the EDS technique . X-ray photoelectron spectroscopy (XPS) may be necessary to detect these AlN particles since it is a more surface-sensitive technique. XPS was not available to the laboratory for this analysis.
General chemical analyses by optical emission spectroscopy and Brinell hardness testing of the castings indicated they met the requirements. There were no chemical requirements for nitrogen and aluminum contents in the proprietary specification. Charpy impact testing of several castings indicated low absorbed energy at − 40 °C (4–8 joules) and 100% brittle fracture surface, correlating with the tensile test results.
Aluminum nitride embrittlement of heavy steel castings has been identified as a mechanism since at least 1947, as published by Lorig and Elsea [3, 5]. The embrittlement shows up as poor tensile ductility and poor impact properties , but it is important to note that tensile strength and yield strength may still meet the required minimums of the ordering standard . Another important point is that keel blocks tested from the same pour will likely not exhibit any problems because they are typically 25–32 mm wall thickness, resulting in much faster cooling rate than the bulk casting; in this way they are not representative of the casting for embrittlement assessment purposes [5, 7]. The embrittlement can appear in normalized structures, quenched and tempered martensitic microstructures [2, 3], martensitic stainless steels , and particularly as cracking in the heat affected zones of welded steel castings , as was observed in this work.
Aluminum nitrides are known to precipitate on the primary austenite solidification grain boundaries under certain conditions and provide a brittle pathway for cracking [2, 3, 4, 5, 7]. The principal variables affecting the embrittlement mechanism are cooling rate, aluminum content, and nitrogen content [2, 3, 4, 5, 7]. It is also known that a similar embrittlement can be active in steel by precipitation of aluminum borides, aluminum carbo-borides, alloy carbides, or these precipitates combined with ferrite at the grain boundaries [2, 3].
Aluminum is present in the steel normally as a residual from the deoxidation practice, and nitrogen can easily be dissolved in liquid steel from an uncontrolled atmosphere . Aluminum nitrides precipitate at the primary solidification grain boundaries with C-curve kinetics, similar to chromium carbide precipitation (i.e., sensitization) in austenitic stainless steels or like pearlite formation in carbon steel normalization [5, 7], hence the importance of cooling rate. The nose of the AlN precipitation curve is at approximately 1150–1250 °C with a critical cooling time of hundreds to thousands of seconds, depending on steel composition and residual aluminum and nitrogen contents [5, 7]. Increasing aluminum content at a fixed nitrogen level decreases the time for precipitation [5, 7]. Once formed, AlN precipitates are not believed to dissolve in steel appreciably until above 1250 °C; this explains why normal heat treatments of steels do not remove the embrittlement condition once developed .
It was found in green sand mold casting that limiting the residual aluminum content to < 0.08 wt.% and wall thickness to < 102 mm generally prevented AlN embrittlement with standard practices [5, 7]. However, AlN embrittlement has been observed in larger castings with 0.02–0.06 wt.% aluminum, with reported nitrogen contents from 36 to 176 ppm [2, 3, 6], emphasizing the importance of cooling rate and that prevention cannot be achieved by chemistry alone.
The cooling rate of a casting is controlled by its size, its surface-area-to-volume ratio, the sand type, and casting rigging configuration . Casting practice in the past, using green sand molds, was empirically developed to achieve at least the critical cooling rate for typical aluminum and nitrogen contents [5, 7]. More formal Hannerz charts showing acceptable cooling rates for avoidance of AlN embrittlement as a function of nitrogen and aluminum content were developed; however, these charts have been found to be non-conservative, and many foundries do not know or actually measure cooling rates . In addition, nitrogen content is often not monitored because the analyzers are expensive and the analysis is too time-consuming compared to the desired production schedule .
With the development of air-set sand molding using organic binders, the actual cooling rates in more recent practice have slowed. The air-set sands are more desirable because they provide for better cast dimensions, cleaner surfaces, and more accurate reproduction of fine details [5, 7]. The slower cooling rates may be due to volatilization of the organic binders during pouring, which provide insulating vapor spaces in the mold walls [5, 7], and possibly due to exothermic reactions in these organically bonded sands . The older, green sand molds are clay-bonded and provide greater thermal conductivity and higher cooling rates.
Given the above, an immediate question is how are nitriding steels successfully processed? These steels, such as the Nitralloy alloys, have intentional aluminum contents around 1.0 wt.% expressly for the purpose of combining with nitrogen for case hardening . Obviously, the nitrogen for case hardening is applied after forming, but one would expect significant residual nitrogen from the ingot casting process, as in any other steel, such that AlN embrittlement would be of concern. Ingots of these nitriding steels are most likely (1) homogenized after casting at high enough temperature to dissolve and break up the primary austenite AlN network, (2) continuously cast in sufficiently small cross sections to allow for fast cooling, or (3) hot rolled at temperatures where hot ductility can counteract the effects of any embrittlement as the AlN network is mechanically broken up.
Considering the foregoing history of aluminum nitride embrittlement, dozens of Type 6 and Type 9 castings from different heats were analyzed for nitrogen content, aluminum content, and degree of brittleness. Tensile specimens were prepared and tested per ASTM A 370. The elongation to fracture in a standard gage length was used as a measurement of degree of embrittlement. In addition, a secondary evaluation was developed by estimating % brittle fracture appearance, similar to the % shear evaluation in ASTM E 23 for impact specimens. In parallel, the ASTM A 703 macro-etch test was conducted for correlation with the data.
The castings fractured due to aluminum nitride embrittlement in combination with the welding thermal expansion/contraction and phase transformation stresses. The three main factors influencing aluminum nitride embrittlement are aluminum concentration, nitrogen concentration and the cooling rate of the casting. The data presented here suggest nitrogen content is less influential than the other two variables. Aluminum has been suggested to be controlled to 0.08 wt.% or less to prevent AlN embrittlement; this recommendation is supported by the data presented in this article. However, care must be taken in making this recommendation to foundries, since cooling rate cannot be neglected.
The severity rating by the macro-etch supplementary requirement S23 in ASTM A 703 has been available for decades to assess heavy castings for AlN embrittlement, and it can be applied to smaller steel castings. This work showed that it is a reliable way of screening castings and illustrated trends in brittleness better than chemistry alone. It should be used as a purchasing requirement in any situation where cooling rates are not known or controlled, or if aluminum or nitrogen concentrations are suspected to be uncontrolled.
It has been suggested that AlN embrittlement may be avoided by titanium additions, since Ti has a higher affinity for nitrogen and would bind it up in more favorable morphology precipitates. Another suggestion has been conducting a homogenizing heat treatment at approximately 1316 °C for 3–4 h after the casting has cooled . The effectiveness of these recommendations could not be assessed within the scope of this project.
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