Abstract
This paper examines ASTM A768/A768M-1995, which is a standardized specification for gas turbine rotor and shaft forgings made from 12% chromium alloy steel that is vacuum processed. The paper not only reviews the mechanical properties found in ASTM A768/A768M-1995, but also compares and contrasts the properties with those of other similar materials used in gas turbine components. Additionally, the paper reviews some benefits of using 12% chromium alloy steel in producing gas turbine rotors and shafts. Finally, the paper recommends using this material in the creation of gas turbine rotors and shafts due to its ability to meet or exceed the requirements of ASTM A768/A768M-1995 and its durability in high-temperature applications.
Introduction
Gas turbines are machines that use combustionand the resulting pressurized gases to turn a turbine and generate mechanical or electrical power. Gas turbine components are often made from materials that must have both sufficient strength to withstand the core strength of the turbine, and an ability to resist the high temperatures that develop in these systems. ASTM A768/A768M-1995 is a standardized specification that sets forth mechanical requirements and testing procedures for 12% chromium alloy steel forgings that are used in the construction of certain gas turbine rotor and shaft components. This paper will examine ASTM A768/A768M-1995, analyze the mechanical properties found in the specification, review some advantages that 12% chromium alloy steel provides in the manufacture of gas turbine bolts, discuss some of the similarities and differences between ASTM A768/A768M-1995 and other material standards applied to gas turbine components, and finally, make a recommendation for using 12% chromium alloy steel forgings according to ASTM A768/A768M-1995.
ASTM A768/A768M-1995
ASTM A768/A768M-1995 is the product of the American Society for Testing and Materials (ASTM). This specification describes the requirements for 12% chromium alloy steel forgings for gas turbine rotors and shafts that have undergone vacuum processing. The mechanical requirements specified in ASTM A768/A768M-1995 are as follows:
Tensile Strength – ASTM A768/A768M-1995 specifies that the 12% chromium alloy steel forgings must meet a minimum tensile strength of at least 260,000 psi.
Yield Strength – The specification also states that the material must have a yield strength of not less than 95,000 psi.
Elongation – The standards minimum elongation requirement is 20% in 10in gauge length.
Brinell Hardness – Brinell hardness shall be 1125, per ASTM E 10-85.
Fatigue Strength
Fatigue strength, as identified in ASTM A768/A768M-1995, must be 20,000 psi in a 180° minimum bend radius.
Comparisons to Other Materials
There are other materials which can be used in the manufacture of gas turbine rotors and shafts. Materials such as Stainless Steels, Nickel-based Alloys, Aluminum-based Alloys and Titanium-based Alloys may also be used. Stainless Steels and Nickel-based Alloys are both strong and able to resist very high temperature applications. Aluminum-based Alloys are strong and light, while Titanium-based Alloys are very strong and light and possess excellent corrosion resistance.
However, the 12% chromium alloy steel specified in ASTM A768/A768M-1995 has certain advantages over many of these materials. Since 12% chromium alloy steel is stronger than Stainless Steel and Nickel-based Alloys, it provides greater strength for the same weight and size. Additionally, since the material has very good corrosion resistance, it may be used in applications where corrosion is a factor.
Finally, 12% chromium alloy steel has a high strength-to-weight ratio compared to Aluminum-based Alloys and Titanium-based Alloys, and its tensile strength, yield strength and fatigue strength outperform the other materials.
Advantages of 12% Chromium Alloy Steel
The use of 12% chromium alloy steel in the manufacture of gas turbine rotors and shafts presents several advantages. First, it has excellent hot and cold workformability, which makes it easier to form and shape the material. Second, the material has very good fatigue strength, which helps to ensure that the components will remain intact even when subjected to cyclical and vibrational loading. Lastly, due to its very high tensile and yield strength, it is able to resist even the most extreme loads and temperatures encountered in gas turbine applications.
Conclusion
According to ASTM A768/A768M-1995, 12% chromium alloy steel forgings meet or exceed the minimum requirements outlined by the specification. Furthermore, the material has many advantageous properties, such as its high strength, excellent hot and cold workformability and its fatigue strength. Consequently, 12% chromium alloy steel is well suited for use in the manufacture of gas turbine rotors and shafts, and therefore, it is recommended that this material be used in creating such components.
