Q: A customer has asked whether we can supply carbon and alloy steel valve bodies meeting Supplemental Requirement S23 in ASTM A What is the. fication, shall apply to steel castings for pressure-containing Specification A and SI units for material ordered to parts under each of the following ASTM. Buy ASTM A /AM: SPECIFICATION FOR STEEL CASTINGS, GENERAL REQUIREMENTS, FOR PRESSURE-CONTAINING.
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What is the purpose of this requirement? The macroetch test specified in Supplemental Requirement S23 in ASTM A is designed to detect the occurrence of aluminum nitride embrittlement in castings.
When steel is melted, oxygen and nitrogen astj 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.
Supplemental Requirement S23 in ASTM A
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 satm 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. Most foundries have the equipment necessary to analyze and report 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.
Certain elements are known to inhibit the formationof aluminum nitride. It has also been found thathigher levels of phosphorus, sulfur,boron, molybdenum and copper inhibitaluminum nitride embrittlement. 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.
Standard Specification for Steel Castings, General Requirements, for Pressure-Containing Parts
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 awtm rate. For example, air-set sand is known to inherently produce molds that cool slower than green-sand molds. For heats with aluminum content greater than 0. The resulting etched specimen is compared with 10 reference macrographs.
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Variables That Influence Embrittlement There are a number of variables that influence the potential for this type of embrittlement: A cross-section of the heaviest section of a sacrificial casting A coupon attached to the heaviest section A coupon removed asfm 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.
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