On this page 10 sections
What is an H-grade stainless steel?
Short answer
An H grade, such as SS 304H, SS 321H or SS 347H, has carbon held between 0.04 and 0.10% and a coarse grain size, which together give reliable creep-rupture strength above about 500 °C. H grades are specified for boiler tubes, heater tubes and high-temperature pressure piping.
What makes an H grade
H vs L vs standard
L grades trade strength for weld corrosion resistance, while H grades trade corrosion resistance for high-temperature strength. Dual-certified SS 304/304H or SS 316/316H material is common for elevated-temperature piping.
Applications
Where H grades go
- Superheater and reheater tubes in power boilers
- Fired heater tubes in refineries and petrochemical plants
- Hot process piping and headers
- Expansion bellows and high-temperature ducting
Ordering mistakes to avoid
Ordering plain SS 304 or SS 347 for creep-range parts, omitting the grain-size requirement, or accepting a certificate without the solution-treatment temperature. The SS 304H boiler tube checklist lists what to include.
Materials in this guide
Related guides, comparisons and articles
Frequently asked questions
Are H grades harder to weld?
They weld much like standard grades, but their higher carbon makes them more prone to sensitisation, so corrosion-sensitive parts may need stabilised H grades.
Which is strongest in creep, SS 304H, SS 321H or SS 347H?
SS 347H generally has the highest creep strength of the three, followed by SS 321H and SS 304H, depending on temperature.
Can SS 304L be used at high temperature?
It can be used, but it has lower allowable stresses in the creep range than SS 304H, so designs need thicker walls. H grades are preferred for pressure parts above about 500 °C.
Are corrosion indices enough to select a material?
No. Indices such as PREN rank alloys, but temperature, crevices, flow, contaminants and fabrication quality all matter. Use service experience, corrosion data or testing for critical applications.
Does Bright Tubes supply the materials in this guide?
Yes. SS 304H Stainless Steel, SS 321H Stainless Steel, SS 347H Stainless Steel, P91 Alloy Steel (Modified 9% chromium-1% molybdenum-vanadium), Incoloy 800HT and related products are supplied from stock or mill-sourced, with EN 10204 certificates.