I am writing this blog after the Steelers defeated the Jets for the AFC Championship and a place in the 2011 Superbowl. The Steelers have won 6 Superbowls, played in 7 Superbowl games, and won 7 AFC Championships. I know all the players’ names and their origin. But do you in the turbomachinery industry know where your bolts came from, especially their origin, place of manufacture, etc.?
Material for the bolts can come from anywhere in the world and providing a USA supplier can perform tasks such as cutting of threads, etc., the bolts can be declared as ‘Made in the USA’. In the turbomachinery industry, the bolt material and how it is processed are important criteria in producing acceptable quality bolts. Bolts are as important as components made from plate, castings, and forgings that makeup the various pressure containing casings for compressors and steam turbines. These bolts connect all the components together and are vital for the smooth and safe operation of machines in the turbomachinery industry.
Throughout the past 20 years, numerous changes have been made in the manufacture of bolts, as companies look for new ways to reduce costs and improve on delivery. Such changes have been the introduction of induction heating into the specification ASTM A193 Grade B7. By applying this quick method of heat treating bars used in the manufacture of bolts, all of the properties can be achieved including the 0.2% proof stress minimum values. However, there is a difference in the values of the limit of proportionally of bars heat treated by conventional methods verses induction.
For low alloy steels heat treated by conventional methods gave the 0.2% proof and limit of proportionality about the same value. However, for some low alloy steel heat treated by induction heating, the limit of proportionality can be 60% to 70% of the 0.2% proof stress. The limit of proportionality term means the greatest stress that can be applied to an elastic body without causing permanent deformation. For alloy steels, this reduction can be a concern, especially for applications which see higher temperatures causing premature stress relaxation of the bolts leading to leakage at the join interfaces of machines. In some cases, these machines can carry or process lethal gases.
You need to know your bolts. Do you? As stated previously, bolts can be made anywhere in the world; melted in Brazil, manufactured as bars in China or India, shipped to the USA for final machining of the bolts, i.e., ‘Made in the USA’ The ASTM specifications, such as ASTM A193, specifies various melting options ranging from open hearth melting to vacuum induction melting. The choice is yours or you can leave it up to the manufacturer. The vacuum degassing process is not mandatory and the ASTM specification leaves it up to the manufacturer of the bolt or the purchaser. At the Elliott Group, for bolts that connect pressure containing components, we specify the melt practices to ensure that the material meets the required chemistry and microstructure integrity. Do you?
At Elliott Group, we know where our bolts came from and how they are manufactured. Do you?