Yes, SA 387 Grade 12 Class 2 is magnetic. This chromium-molybdenum alloy steel plate, specified by ASME SA-387 and its identical ASTM counterpart A387, has a microstructure of ferrite, bainite, or tempered martensite, all of which are ferromagnetic. A magnet will therefore be attracted to the plate in the same way it is attracted to ordinary carbon steel. The magnetic property is a normal characteristic of the material and has no adverse effect on its service performance. This guide explains the reason for the magnetism and its practical implications, and reviews the properties and applications of the grade.
Why SA 387 Grade 12 Class 2 Is Magnetic
Magnetism in steel depends on the crystal structure of the alloy. SA 387 Grade 12 Class 2 is a low-alloy chromium-molybdenum steel containing about 1 percent chromium and 0.5 percent molybdenum. Because the chromium content is far below the level required to form an austenitic structure at room temperature, the steel solidifies and transforms to body-centered cubic ferrite and related phases during rolling and heat treatment. These phases are ferromagnetic, meaning the material contains magnetic domains that align with an applied magnetic field. The magnetic permeability of the grade is close to that of plain carbon steel, so standard lifting magnets and magnetic clamping devices work normally with it.
Material Overview and Key Properties
Grade 12 Class 2 is supplied in the normalized and tempered condition. The specified tensile strength is 65 to 85 ksi (450 to 585 MPa), the minimum yield strength is 40 ksi (275 MPa) at 0.2 percent offset, and minimum elongation is 19 percent in 8 inches and 22 percent in 2 inches. The heat analysis composition is carbon 0.05 to 0.17 percent, manganese 0.40 to 0.65 percent, phosphorus and sulfur 0.035 percent maximum each, silicon 0.15 to 0.40 percent, chromium 0.80 to 1.15 percent, and molybdenum 0.45 to 0.60 percent. The grade offers good high-temperature strength, creep resistance, and oxidation resistance, which is why it is specified for elevated-temperature pressure vessel service.
Practical Implications of the Magnetic Property
The ferromagnetic behavior of the material has several practical consequences. Magnetic particle testing, one of the standard non-destructive examination methods for ferromagnetic steels, can be applied directly to the plate and to completed welds, which is convenient for quality control during fabrication. The steel can be handled and positioned with magnetic lifting devices in the workshop. The magnetic property also provides a simple way to distinguish this low-alloy steel from austenitic stainless steels, which are non-magnetic, helping to prevent material mix-ups in storage and fabrication. None of these effects impairs the mechanical or corrosion performance of the plate in its intended elevated-temperature service.
Fabrication and Welding
Welding of SA 387 Grade 12 Class 2 requires qualified procedures. Preheating is normally required, typically in the range of 150 to 200 degrees C, to avoid hydrogen-induced cracking in the heat-affected zone. Low-hydrogen filler metals matching the base metal chemistry, such as E8018-B2 type electrodes for shielded metal arc welding or ER80S-B2 filler for gas tungsten arc welding, are commonly used. Interpass temperature should be controlled so that it does not exceed the limit of the procedure specification, usually about 300 degrees C. Post-weld heat treatment is generally required after welding and is typically performed at 650 to 700 degrees C, with the holding time determined by plate thickness.
Typical Applications
The grade is used in oil refineries and petrochemical plants for reactor vessels, separators, and storage tanks operating at elevated temperature, and its chromium content makes it suitable for service involving hydrogen at high temperature. In power generation it is applied to boiler drums, steam pipelines, and heat recovery steam generator components. Pressure vessel fabricators use it for welded vessels and industrial boilers where internal pressure and temperature exceed the limits of carbon steel. The material is also processed into flanges, fittings, and high-temperature piping components. It is not intended for cryogenic service, since its toughness decreases at very low temperatures.
Frequently Asked Questions
Is SA 387 Grade 12 Class 2 magnetic?
Yes. The steel has a ferritic, bainitic, or martensitic microstructure, which is ferromagnetic, so it is attracted by magnets like ordinary carbon steel.
Why is this steel magnetic when some stainless steels are not?
The chromium content of about 1 percent is too low to stabilize an austenitic structure, so the material retains a ferromagnetic ferritic structure, whereas austenitic stainless steels with much higher alloy content are non-magnetic.
Can a magnet be used to lift SA 387 plates?
Yes. The magnetic permeability is close to that of carbon steel, so standard magnetic lifting and clamping equipment works normally with this material.
Does the magnetic property affect pressure vessel performance?
No. Magnetism is a normal physical property of the steel and has no effect on strength, toughness, creep resistance, or corrosion resistance in service.
Is magnetic particle testing possible on this material?
Yes. Because the steel is ferromagnetic, magnetic particle testing can be applied to the base plate and welds, which is one of the standard NDT methods used during fabrication.
What is the composition of Grade 12 Class 2?
The main alloying elements are chromium at 0.80 to 1.15 percent and molybdenum at 0.45 to 0.60 percent, with carbon 0.05 to 0.17 percent and manganese 0.40 to 0.65 percent.
What temperature range is the grade suitable for?
It is intended for elevated-temperature service; its toughness decreases at very low temperatures, so it should not be used for cryogenic applications, and the upper temperature limit is set by the design code based on creep and oxidation behavior.