Heat-Resistant Alloy Service Life Extended Through Alloy Development
Standard heat resistant alloys (HRAs) have been the basic workhorse alloys used to manufacture fixtures, radiant tube assemblies and high-temperature operating components in different furnace designs for many years, and are still performing well in some heat-treating applications. However, these alloys often experience shorter than expected service lives with the increasing demands of running more efficient heat-treating operations by optimizing heat-treated loads and seeking higher operating temperatures in most applications.
Short service lives of furnace parts made of standard HRAs are attributed to many factors inherent in these alloys, including relatively low creep strength, low corrosion and oxidation resistance and poor resistance to distortion. These factors force heat treaters to perform more unscheduled furnace maintenance to replace failed components. In many heat-treating operations, once a furnace is shut down, all the heat-resistant furnace components, as well as the refractory, are replaced to minimize further component failures and subsequent unscheduled shut downs.
Standard HRAs, which basically are composed of chromium, iron and nickel, have not undergone any significant changes to their chemistries since their introduction to the heat treat industry. However, alloying technologies have been developed and used in the formulation of new alloys, which have enhanced high-temperature mechanical properties and corrosion behavior. Limited research has left standard HRAs stagnant and limited their applications.
Standard HRA strengthening mechanisms
In the temperature range 1600 to 2010F (870 to 1100C), the main strengthening mechanism that operates in standard heat resistant alloys during the steady state creep process is the result of carbide dispersion. Increasing temperature and/or long exposure at high temperature affect the growth and morphology of the carbide (and sometimes nitride) precipitates in these alloys. Although the precipitation of secondary carbides does not start until around 1100F (595C), their morphology is greatly affected in the temperature range considered. The austenitic matrix, having a wide solubility range for carbon, becomes a field of carbide precipitates at high temperature. Carbon reacts readily with the chemical elements that are strong carbide formers, such as chromium and tungsten. Carbide precipitates are very effective in their interaction with dislocations as long as their critical size is not reached. The effectiveness of this interaction strengthens these alloys and imparts resistance to creep and plastic deformation at high temperature.
Once carbides grow past a certain critical size, their interaction with dislocations becomes less effective and, consequently, the alloys start losing their strength. Carbide precipitates grow not only by diffusion processes, but also by coalescence with one another. The higher the temperature, the more advanced their growth and coalescence processes become.
Less secondary, but bulky carbide precipitates are present in the matrices of grains. Carbon and chromium eventually diffuse out of the grain matrices and settle in interdendritic and grain boundary regions, which become heavily carburized. At this stage, the alloy becomes vulnerable to degradation of its mechanical properties. The matrix is depleted of carbon and chromium, thereby losing both its solid-solution and interstitial solution strengthening. However, grain boundaries and interdendritic regions become filled with coarse, bulky carbides, degrading the ductility of the alloy at these sites. Carbides, being ceramic compounds and having different thermal expansion coefficients than the metallic matrix, start to crack and also crack the grain matrices once they reach their coarsened bulky sizes. This occurs during furnace process heating and cooling cycles and during thermal fluctuations.
By comparison, the special formulation of chemical composition and casting processes of the Advanced Alloys results in microstructural characteristics that retards the growth of the primary and secondary carbide precipitates. Consequently the carbon and chromium stay in the matrices of the grains longer, maintaining alloy high-temperature strength for a much longer time. Therefore, the new alloys are more resilient to creep and deformation processes than the traditional alloys.
Tensile and stress-rupture specimens were produced at Steeltech Ltd. in accordance with ASTM E-8 (Standard Test Methods for Tension Testing of Metallic Materials). Specimens were machined from 1.25 in. wide by 6 in. long coupons cut from the bottom of the keel-blocks and inspected to ensure there were no surface defects. Yield stress, ultimate tensile stress and percent elongation were measured in room-temperature tensile tests, and stress rupture tests were performed at temperatures of 1600, 1800 and 2010F (870, 980 and 1100C) for alloys AA51, AA65 and AA41. The stresses used for these tests were selected to draw a direct comparison with standard heat resistant alloys HT, HK, HP, and HH. Most of the testing was performed at Howmet Whitehall Research Center, Whitehall, Mich. Room-temperature testing
To give physical meaning to the comparison that is established between the Advanced Alloys and some of the standard heat resistant alloys discussed in this article, two testing conditions were selected from published and commonly used standard HRA data.
The mechanical properties of AA51 and published data for the heat resistant alloys HK, HT[1, 2] are compared in Table 1. The yield stress, ultimate tensile stress and elongation are similar at room temperature. Tensile properties of AA 65 and standard heat resistant alloy HP also are similar in magnitude (Table 2), and differences between tensile properties of AA 41 and HH2 are negligible (Table 3).
To produce quicker testing results, load and temperature conditions were selected from the stress-rupture test conditions of 100 hour average duration for the standard heat resistant alloys.
AA51 has two to ten times the performance of HK and HT under the same test conditions. Test results for HK, HT and AA51 are presented in Table 4. At 1600F (870C) under a stress of 9,200 psi, both HK and HT alloys last on the average of 100 hours, then fail catastrophically compared with a life of 210 to 290 hours for AA51. At 1800F (980C) under a stress of 4,750 psi, both HK and HT fail catastrophically after an average of 100 hours compared with 325 to 770 hours for alloy AA51. At 2010F (1100C), HK and HT have a 100-hour life at a stress of 2,200 psi compared with 450 to 1,050 hours for AA51.
AA65 has three to four times the performance of HP under the same test conditions. Stress-rupture results for HP and AA65 are shown in Table 5. HP has an average 100-hour life at 1600F under a stress of 10,000 psi compared with 315 to 470 hours for AA65. The life of AA65 is between 200 and 250 hours at 1800F and 5,900 psi and between 300 and 405 hours at 2010F and 2,800 psi compared with a 100-hour average life of HP at the same conditions.
AA41 has two to ten times the performance of HH under the same test conditions. Table 6 lists the results of stress rupture tests HH and AA41. HH has an average 100-hour life at 1600F under a stress of 6,400 psi compared with 210 and 350 hours for AA41. The life of AA41 is between 350 to 670 hours at 1800F and 3,200 psi and between 829 to 1,007 hours at 2010F and 1,400 psi compared with a 100-hour average life of HH at the same conditions.
The long stress-rupture lives that characterize the Advanced Alloys are due to their high creep strength, which is directly due to the special chemical formulation and special casting procedures used to make these alloys. This results in a fine primary carbide network dispersion in the as-cast condition.
Advantages of Advanced Alloys to a heat-treating operation
Extending the service lives of furnace hardware offers significant benefits to the production, maintenance and efficiency of a heat-treating operation. For example, two to ten times longer service life than those for traditional HRAs eliminates many shut downs and reduces unscheduled down time, which results in increased production. In addition, reducing furnace-hardware failures leads to fewer purchases, reduces furnace-parts inventory and achieves a more efficient heat-treating operation.
The following examples illustrate the extended performance using Advanced Alloys:
- "U" radiant tube assemblies made of a standard HRA in the harsh environment of the first zone of a continuous carburizer at a Caterpillar plant previously were lasting about 1-1/2 years, typically failing due to sagging, blow holes and weld failures. By comparison, Advanced Alloy AA51 U-tube replacements have been in service for nearly three years without any signs of deterioration.
- "I" beams made of AA51 show no signs of deterioration after nearly 3-1/2 years in furnaces at Hansen Balk Steel Treating Co. compared with an average service life of 1-1/2 years for beams made of a standard HRA.
- Roller rail sets made of standard HRAs had an average service life of one year in a high carbon atmosphere at a temperature of 1750F (955C) in a furnace at Thermal Metal Treating Inc., typically failing due to warping, bowing and cracking. Advanced Alloy A51 roller rails, by comparison, show no signs of failure after nearly 2-1/2 years.