Michigan Seamless Tube - Making The Grades
FIGURE 1 Thermal profile generated VS.
requirement of 1,950º for 20 minutes.
Here’s what one company went through to adapt to two new steel opportunities.
Michigan Seamless Tube (MST) strives to be the domestic source for all of the boiler industry’s carbon and alloy steel tubular needs. With the recent development by the industry of special heat treatment requirements for grade 91, and the addition of the new steel grade 23, MST has been actively involved in updating its capabilities in order to continue supplying these grades.
All of MST’s raw material is purchased from domestic mills. They have partnered with two key suppliers, and their combined efforts enable them to supply all of the industry’s carbon and alloy steel needs as 100% melted and manufactured in the U.S. Once the bar is received by MST, it is cut to the necessary length dictated by the customer order and then pierced at the manufacturing facility located in South Lyon, MI.
FIGURE 2 Grade 91 Microstructure after
tempering 100x.
The industry’s two newest creep strength enhanced ferritic steels, grades 91 and 23, both require special heat treatment of the finished tubing. The process required higher temperatures and longer cycle times than MST was capable of performing with its in-house annealing equipment. Therefore, the company began a search nearly two years ago to find a vendor capable of providing these special heat treatments.
During the nearly four-month search, over 30 potential heat treat vendors were identified. After a preliminary assessment of their capabilities, five potential heat treat vendors were visited. The visits enabled MST to determine the vendors’ true capabilities, assess their quality systems, ensure that adequate production capacity existed, and quantify their interest in developing a partnership. After this exhaustive search process, MST decided to partner with Franklin Brazing and Metal Treating.
TEMPER, TEMPER
The required process setup was qualified over several small quantity trials, wherein MST’s tubular product was heat treated in a very controlled environment. Franklin Brazing uses a "pure atmosphere" system composed of cryogenic liquid hydrogen/nitrogen to ensure that the tubular product is protected throughout the process from carburization, decarburization, and oxidation. These initial trials involved optimizing the process - time, temperature, and speed - to ensure that the tubing received the required thermal profile for both heating and cooling throughout the entire run, completely through the tube wall thickness, and fully along each tube length.Thermocouples were inserted into holes that were drilled longitudinally, starting on the face of the end of the tube and going several inches deep, then into the length of the tube. These drilled tubes with thermocouples were then placed on both outside edges of the furnace charge and in the center to ensure uniform heating and cooling within each group of tubes as they traveled through the furnace. Several such trials were performed on tubing of various diameters and wall thicknesses, and thermal profiles were generated using the output from the thermocouples to determine an optimal process for each possible OD and wall combination. The thermal profiles generated were very consistent and verified having met the time and temperature requirements of the specified process.
The tensile testing and microstructure analyses were completed by MST using its own laboratory and metallurgists, and creep testing was completed in conjunction with an independently contracted lab. During the trial period, both hardness and microstructure were used to determine whether the material was acceptable for tempering. Hardness measurements after normalizing are now used as the criteria to determine whether or not the material is acceptable for tempering. All material that has been run has met the minimum hardness requirements.
Table 1
After determining the optimal setting for a given size of tubing, additional trials were then run and samples were cut for testing to positively ensure that both microstructure requirements and tensile properties were being met. Several trials and subsequent testing provided sufficient data to confirm a stable and conforming process had been established, so additional tests consisting of long cycle creep testing were undertaken for grade 91. Samples were cut and sent out for stress rupture testing (creep) to evaluate the mechanical behavior of the material. Each of the samples that have been tested exceeded both the minimum test requirement and the mean life requirement. TB
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