What JIS G3462 Covers
JIS G3462 is the Japanese Industrial Standard for alloy steel pipes for boilers and heat exchangers. It covers seamless tubes that must carry steam, water or hot process fluid at elevated temperature and pressure, and it is written as a companion to JIS G3461, which covers the equivalent carbon steel product. The alloy steel grades in JIS G3462 contain molybdenum, or chromium and molybdenum, so that they retain strength at temperatures where plain carbon steel would creep or scale excessively.
The prefix STBA identifies an alloy steel boiler and heat exchanger tube. The number that follows the prefix identifies the grade and therefore the alloy level, so STBA 12 is the molybdenum steel of the series, containing molybdenum within the range stated in the standard.
Chemical Composition of STBA 12
| Element | Requirement, mass percent |
|---|---|
| Carbon | 0.10 to 0.20 |
| Silicon | 0.10 to 0.50 |
| Manganese | 0.30 to 0.80 |
| Phosphorus | 0.035 maximum |
| Sulphur | 0.035 maximum |
| Molybdenum | 0.45 to 0.65 |
The controlled molybdenum range is the defining feature of the grade. Molybdenum raises the creep rupture strength of ferritic steel and improves its resistance to hydrogen attack, which is why the 0.5 % molybdenum type has been a standard boiler material for decades. Low phosphorus and sulphur are important because these elements segregate at grain boundaries and reduce toughness and creep ductility in long term high temperature service.
Mechanical Properties and Heat Treatment
| Property | Requirement for STBA 12 |
|---|---|
| Tensile strength | 380 MPa minimum |
| Elongation, A5 | 30 % minimum |
| Yield strength | Specified in JIS G3462 for the grade and confirmed on the mill certificate |
| Flattening and bend tests | Required by the standard on tube samples |
| Hydrostatic test | Applied to each tube unless an agreed non-destructive test replaces it |
Tubes are supplied in the heat treatment condition specified by the standard, normally normalized or as agreed between the mill and the purchaser, and the surface may be supplied as hot finished or cold finished depending on the finish required by the heat exchanger design.
Service Conditions and Applications
Boiler tubes, water wall tubes and economizer tubes in steam generating plant.
Superheater and reheater tube elements operating with metal temperatures up to about 500 °C.
Shell and tube heat exchanger tubes in refineries, petrochemical plants and power stations.
Steam headers, connection piping and other pressure parts where a low alloy tube is required by the design code.
The grade is chosen for the lower end of the alloy boiler tube range. Where metal temperature or pressure exceeds its capability, a chromium-molybdenum grade with higher creep strength is specified instead.
Fabrication and Quality Points
Cold bending, hot bending and tube expanding are all used in boiler assembly; the bend radius and the resulting wall thinning must be checked against the design allowance.
Welding should use a filler metal of matching or higher alloy content, with preheat only where the wall thickness and the welding procedure require it, followed by the post weld heat treatment specified for the assembly.
Cold work followed by high temperature service can cause graphitisation in some low alloy steels, so severe cold forming is normally followed by annealing before the tube enters service.
Ordering should state the standard, grade, outside diameter, wall thickness, length, finish, heat treatment condition and the testing required, so that the mill certificate and the design calculation agree.
FAQ
Q: What is the difference between JIS G3461 and JIS G3462?
JIS G3461 covers carbon steel boiler and heat exchanger tubes with the STB prefix, while JIS G3462 covers alloy steel tubes with the STBA prefix, including molybdenum and chromium-molybdenum grades.
Q: What is STBA 12?
STBA 12 is the 0.5 % molybdenum alloy steel tube grade of JIS G3462, with a controlled molybdenum range of 0.45 to 0.65 % that gives higher creep strength and better resistance to hydrogen attack than carbon steel.
Q: What are the mechanical requirements for STBA 12?
JIS G3462 requires a minimum tensile strength of 380 MPa and a minimum elongation of 30 % on the A5 gauge length, with yield strength and test methods specified in the grade table of the standard.
Q: What is the maximum service temperature of STBA 12?
The grade is used with metal temperatures up to about 500 °C. Above that range the creep strength of a 0.5 % molybdenum steel is no longer adequate and a chromium-molybdenum alloy grade is selected.
Q: How should STBA 12 tubes be welded?
Use a filler metal of matching or higher alloy content, control heat input, and apply the post weld heat treatment required by the construction code. The welding procedure should be qualified for the tube size and wall thickness before production welding starts.
Q: What testing is carried out on STBA 12 tubes?
Testing includes chemical analysis of the heat, tensile and elongation tests, flattening and bend tests on samples, and a hydrostatic test applied to each tube unless an agreed non-destructive examination is substituted under the standard.
