Supplemental Requirement S23 in ASTM A703

A: The macroetch test specified in Supplemental Requirement S23 in ASTM A703 is designed to detect the occurrence of aluminum nitride embrittlement in castings.

When steel is melted, oxygen and nitrogen can be dissolved in the molten steel. When the metal is poured into the casting mold, the oxygen can come out of the solution and form gas porosity in the castings. In order to avoid castings with excessive gas porosity, the foundry may add deoxidizing elements to the melt. The presence of these elements results in the formation of oxide compounds, which mitigates the formation of oxygen gas bubbles during solidification.

One of the elements that is sometimes added as a deoxidizer is aluminum, which creates a potential for the formation of aluminum nitride. This compound tends to precipitate in the prior austenite grain boundaries and can result in castings that display loss of ductility that could result in brittle failure of the casting if it is shock-loaded in service. If fracture occurs, it has a rock-candy appearance due to fracture along the prior austenite grain boundaries. The fracture is often very shiny due to the presence of aluminum nitride platelets on the surface.

Variables That Influence Embrittlement

There are a number of variables that influence the potential for this type of embrittlement:

1) The steel must contain enough aluminum and nitrogen to support the formation of enough aluminum nitride to cause a significant embrittling effect. Most foundries have the equipment necessary to analyze and report aluminum content. However, the specifications for the common cast steels used in valve bodies do not require analysis or reporting of aluminum as a standard requirement, so some foundries do not monitor and/or report the aluminum content.

Although some foundries monitor nitrogen levels in their steel, many do not have the necessary equipment to do this. The specifications for the common cast steels used in valve bodies do not require analysis or reporting of nitrogen as a standard requirement. Therefore, the nitrogen content of the steel is often not known or reported, and it is possible that enough nitrogen is present to form aluminum nitride.

2) Other elements that inhibit the formation of aluminum nitride must be present in low enough quantities that the reaction can occur. Certain elements are known to inhibit the formationof aluminum nitride. Additionsof titanium and/or zirconium, addedeither along with or after the additionof aluminum, can inhibit the formationof aluminum nitride by forming titaniumnitride and/or zirconium nitrideinstead. It has also been found thathigher levels of phosphorus, sulfur,boron, molybdenum and copper inhibitaluminum nitride embrittlement.

3) The cooling rate in the mold must be slow enough through the critical temperature range to allow the formation of the aluminum nitride phase. The thickness of the casting (orportion of the casting) is an obviousfactor influencing whether embrittlementoccurs, since thicker casting sectionswill cool more slowly through thecritical temperature range than thinsections.

The casting configuration can also influence the cooling rate. A complex casting with relatively thin walls may experience slower cooling rates than a simple casting with the same wall thickness because the complex casting will lose heat through the mold less effectively.

The mold material can also influence the cooling rate. For example, air-set sand is known to inherently produce molds that cool slower than green-sand molds.

Reporting Aluminum Content

When Supplemental Requirement 23 in ASTM A703 is imposed on steel castings produced under ASTM A216, A217 and A352, aluminum is required to be reported. For heats with aluminum content greater than 0.08%, a macroetch must be performed on one of the following:

• A cross-section of the heaviest section of a sacrificial casting
• A coupon attached to the heaviest section
• A coupon removed from directly under a riser or
• A coupon from the same heat of material removed from a separately cast test block with a thickness representative of the heaviest section of the casting.

The resulting etched specimen is compared with 10 reference macrographs. Heats with severity levels 4 and lower are acceptable. Heats exhibiting higher levels are dispositioned by a number of means, including examination of etch structures on individual castings, fracture testing to determine the degree of “rock candy” appearance, mechanical testing, weld crack susceptibility testing and/or high-temperature solution heat treatment, as agreed upon by the supplier and purchaser.

REFERENCES:

1. ASTM A703 (latest revision), “Steel Castings, General Requirements, for Pressure-Containing Parts,” ASTM International, West Conshohocken, PA.

2. ASM Materials Information On-Line: ASM Metals Handbook - Volume 1, Properties and Selection: Irons, Steels, and High Performance Alloys -> Embrittlement of Steels -> Embrittlement in Carbon Steels and Alloy Steels.

3. SFSA SFSA Casteel Reporter, August 2006, Page 2, Steel Founders’ Society of America, Crystal Lake, IL.

4. Banks, WC, “Avoiding Aluminum Nitride Embrittlement in Steel Castings for Valve Components,” V-Rep 84-1, Flowserve, 2003, Raleigh, NC.