What ASTM A847 Tubing Is
ASTM A847 is the specification for cold-formed welded and seamless high-strength low-alloy structural tubing with improved atmospheric corrosion resistance. It covers round, square and rectangular shapes, and it is widely used for air heater and air preheater tube bundles, structural frames, handrail and other welded assemblies that see moisture and fluctuating temperature.
The value of the grade comes from two things at once. First, the copper, chromium and nickel additions let the surface develop a protective oxide layer that resists atmospheric attack far better than plain carbon tubing, with or without added copper. Second, the minimum mechanical properties are high enough to carry structural duty without increasing wall thickness: the specification sets a 50 ksi (345 MPa) minimum yield strength and a 70 ksi (485 MPa) minimum tensile strength.
Product Range
| Item | Range |
|---|---|
| Standards | ASTM A847 / ASME SA847 |
| Shapes | Round, square, rectangular; hydraulic and honed tubes also produced |
| Square size | 3 in through 12 in |
| Rectangular size | 4 in x 2 in through 16 in x 8 in |
| Round outside diameter | 6.625 in, 8.625 in, 10.75 in, 12.75 in, 14 in and 16 in |
| Wall thickness | 0.125 in through 0.625 in |
| Length | Single random, double random and cut length |
| End finish | Plain end or beveled end |
Chemical Composition
Two composition classes are defined, Grade 1 and Grade 2. Grade 1 relies on phosphorus plus copper, chromium and nickel for corrosion resistance, while Grade 2 uses a leaner phosphorus level with higher copper and manganese and a molybdenum addition. Both are limited to 0.15 percent maximum carbon so that field and shop welding remain practical.
| Element | Grade 1 | Grade 2 |
|---|---|---|
| Carbon, max | 0.15 | 0.15 |
| Manganese | 0.55 max | 0.50-1.00 |
| Phosphorus | 0.06-0.16 | 0.04 max |
| Sulfur, max | 0.060 | 0.05 |
| Silicon, min | 0.10 | Not specified |
| Copper | 0.20-0.60 | 0.30-1.00 |
| Chromium | 0.24-1.31 | Not specified |
| Nickel | 0.20-0.70 | 0.40-1.10 |
| Molybdenum, min | Not specified | 0.10 |
Where a product analysis is taken from the finished tube rather than from the heat, wider tolerances apply: carbon 0.24 max, manganese 1.40 max, sulfur 0.06 max and copper 0.18 min, while the maximum phosphorus limit is not applied to the product analysis.
Mechanical Properties
| Grade | Minimum yield strength | Minimum tensile strength | Minimum elongation |
|---|---|---|---|
| ASTM A847 | 50,000 psi (345 MPa) | 70,000 psi (485 MPa) | 19 percent |
The combination of a 50 ksi yield and a 70 ksi tensile gives a yield-to-tensile ratio of about 0.71, which leaves useful ductility for cold forming, bending and welded connection design. For air heater bundles the tubing must also tolerate repeated thermal cycling, so the low carbon and fine grain practice matter more than the headline strength figures.
Weld Seam, Corner Radius and Flash Rules
The weld seam of square, rectangular and specially shaped tubing must not be positioned inside the corner radius unless the customer and manufacturer agree otherwise.
Corner radii for square and rectangular tubing are controlled so that the outside radius does not exceed the limits set in the specification.
Electric resistance welded tubing is normally supplied without removal of the internal weld flash. If flash control is needed, it must be stated at the enquiry and order stage, because it changes the production route.
Beveled ends are available for girth welding on site or in the shop; plain ends suit drilled tube sheets and mechanical joints.
Cut lengths should be specified with the tolerance class required by the fabricator rather than with a nominal figure, since thermal cutting and sawing give different end squareness.
Service Notes for Air Heater Bundles
In an air heater the tubes see warm, moisture-bearing gas on one side and cooler air on the other. Sulfur-bearing combustion products condense on cold tube surfaces and form acidic deposits, so the tubing surface condition and the design of the cold end matter as much as the steel grade. Weathering tubing performs best when it can dry out periodically; if it stays wet, corrosion resistance falls back towards that of carbon steel, and a protective coating or a change of grade should be considered. Where tubes are expanded into tube sheets, the yield strength of A847 supports the required pull-out loads without an increase in wall thickness.
FAQ
Q: What is ASTM A847?
ASTM A847 is the standard specification for cold-formed welded and seamless high-strength low-alloy structural tubing with improved atmospheric corrosion resistance, covering round, square and rectangular shapes.
Q: What are the mechanical properties of A847 tubing?
The specification requires a minimum yield strength of 50 ksi (345 MPa), a minimum tensile strength of 70 ksi (485 MPa) and a minimum elongation of 19 percent.
Q: What is the difference between Grade 1 and Grade 2?
Grade 1 uses phosphorus with copper, chromium and nickel for corrosion resistance. Grade 2 keeps phosphorus low, raises copper and manganese and adds a minimum molybdenum content. Selection depends on the corrosion environment and welding requirements.
Q: Can A847 tubing be used for air heater and air preheater tubes?
Yes. The grade combines enough strength for structural duty with a corrosion resistance well above plain carbon tubing, which suits the moisture and acid condensation conditions found in air heater service.
Q: Is the internal weld flash removed?
Electric resistance welded tubing is normally supplied with the internal flash left intact. Removal, or controlled flash height, must be specified at the enquiry and order stage.
Q: How should cut lengths and ends be specified?
State the length class (single random, double random or exact cut), the end finish (plain or beveled) and the required length tolerance, so that the tubes arrive ready for expansion or girth welding.
Q: Does weathering tubing need painting?
Not when it can dry out and form a stable oxide layer. In continuously wet or acid-condensing service, a coating system or a more highly alloyed grade should be reviewed instead.
