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Understanding Effects of High Temperatures on Industrial Grade Bolting Materials

The two most common alloys for industrial grade bolting consist of ASTM A193 B7 grade and ASTM A193 B16 grade. These Chromium-Molybdenum metals share a very common foundation of chemistry but due a small amount of Vanadium in the B16 version, the advantages of using this material at extreme temperatures are significant relative to mechanical integrity. The ASTM specification clearly publishes mechanical properties associated with these two materials and since these values represent characteristics and mechanical properties at ambient temperature, one might assume that these that these are interchangeable.

A review of the chemical content reveals only minor variances in basic composition with the exception of added Vanadium and trace Aluminum to the B16 formulation. While this may seem minor, the effect on integrity and strength at elevated temperature is significant. Vanadium is a steel additive that creates nitrides and carbides in the chemical structure which significantly increases the strength of the steel, particularly at elevated temperatures. The addition of this element results in a bolting material that can better maintain its physical and mechanical integrity at extreme temperatures. ASTM A193 B7 materials are often recognized to be acceptable for use up to 800F service temperature. This is by far the most common industrial bolting solution for general flanges and most fixed equipment applications in a refinery or petrochemical facility where application temperatures approaching this limit are not uncommon. ASTM A193 B16 are often used in temperatures up to 1000F for reasons highlighted in the chart below

Material Strength at Elevated Temperature, graph from John H. Bickford, Introduction to the Design and Behavior of Bolted Joints, Fourth Edition, Taylor and Francis Group 2008

Note that “B17” material represents AISI660 Material and “B80A” represents Nimonic 80 MaterialMaterial Strength at Elevated Temperature, chart from John H. Bickford, Introduction to the Design and Behavior of Bolted Joints, Fourth Edition, Taylor and Francis Group 2008While B7 and B16 materials look very similar in the specification, exploring the effects of temperature on their mechanical integrity can open some opportunity to improve performance in applications that are approaching these elevated temperatures. B16 is recognized to have over 2 times the strength of B7 at these upper limits. While there are limited applications where the external bolting will experience these temperatures, it is an available option which provides an additional safety factor and assurance in high temperature applications.

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