🇨🇳 GB/T 6653—2017 (Official Certification) | Special Steel for Welded Gas Cylinders
HP235 Steel Plates and Strips for Welded Gas Cylinders
Low Carbon · Ultra-low S/P · High Anti-Burst Safety Level
🌐 Authoritative Standard | NDT | Preferred Choice for Global Cylinder Manufacturing
🔐 Core Application Logic: Why Must Gas Cylinders Use HP235?
💡 HP235 ≠ Ordinary low-carbon steel. It is a pressure vessel special steel designed according to GB/T 6653-2017, with every specification engineered for "zero burst" of welded gas cylinders.
- ✔ Low Carbon (C ≤0.16%) → Excellent Weld Crack Resistance
Welded cylinder manufacturing involves extensive girth and longitudinal welding. Excessive carbon equivalent easily produces hardened martensite, leading to cold cracks. HP235 strictly controls carbon content below 0.16%, combined with ladle refining, ensuring HAZ toughness and fundamentally eliminating welding delayed cracks. - ✔ Ultra-low Sulfur & Phosphorus (S≤0.012%, P≤0.025%) → Eliminate Brittle Fracture
Sulfur causes hot brittleness, phosphorus causes cold brittleness (temper embrittlement). Gas cylinders may endure dynamic loads and low-temperature environments during filling and transport. HP235's P/S requirements are far stricter than ordinary structural steel (ordinary steel S≤0.035%), greatly reducing intergranular fracture risk, verified by Charpy impact (room temperature ≥23J, -40℃ optional ≥18J). - ✔ High Elongation (A80mm ≥23%, A≥29%) → Burst Resistance Plasticity
Under pressure overload or fire conditions, cylinders need sufficient plastic deformation capacity to absorb energy rather than brittle bursting. HP235's elongation is more than 30% higher than ordinary structural steel, combined with 180° cold bending (D=1.5a without cracks), ensuring the cylinder leaks and deforms first under extreme conditions, preventing "bomb-like" shattering rupture. - ✔ Grain Size ≥6 + Low Non-metallic Inclusions → Fatigue Life & Low-temperature Toughness
Fine grain strengthening not only improves strength but also enhances low-temperature impact stability. The standard requires grain size not less than grade 6, with non-metallic inclusions (A/B/C/D/DS) strictly controlled to total ≤7.0, avoiding stress concentration sources, allowing cylinders to safely serve over 15 years under repeated pressurization (fatigue) conditions.
⚠️ If ordinary structural steel replaces HP235 for cylinder manufacturing, welding cold crack rate increases by 47%, low-temperature impact qualification rate falls below 30%, and it cannot pass the hydrostatic burst test in cylinder type tests. HP235 is a safety baseline, not an option.
📜 Authoritative Standards & Safety Certification
✅ Based on GB/T 6653-2017 Mandatory Clauses — This standard was proposed by the China Iron and Steel Association and is the only dedicated standard for welded gas cylinder steel plates.
✅ Melting Process: Converter/EAF + LF/VD Refining — Significantly reduces gas content and inclusions, sulfur content can be stably controlled at ≤0.008%.
✅ Non-destructive Testing (NDT) & Pressure Test Adaptation — Ultrasonic flaw detection ensures no delamination, hydrostatic test (2x working pressure) verifies pressure-bearing safety.
✅ Low-temperature Toughness Reserve (-40℃ Impact ≥18J) — Suitable for cylinder projects in severe cold regions, enhancing export competitiveness.
📌 Designation Method (Clause 3)
"HP" = HanPing (welded cylinder), 235 = minimum lower yield strength (MPa). Example: HP235 is clearly distinguished from ordinary commercial steel.
"HP" = HanPing (welded cylinder), 235 = minimum lower yield strength (MPa). Example: HP235 is clearly distinguished from ordinary commercial steel.
🔬 Chemical Composition (Ladle Analysis)
✔ 6.1.1~6.1.6 The steel grade and chemical composition shall comply with Table 1. Residual elements Cr, Ni, Mo each ≤0.30%, Cu ≤0.20%.
| Grade | C≤ | Si≤ | Mn≤ | P≤ | S≤ | Nb≤ | V≤ | Ti≤ | Nb+V≤ | Al≥ |
|---|---|---|---|---|---|---|---|---|---|---|
| HP235 | 0.16 | 0.10c | 0.80 | 0.025 | 0.012 | 0.05 | 0.10 | 0.06 | 0.12 | 0.015 |
📌 Note c: For thickness ≥6mm, Si up to 0.35% permitted. Acid-soluble Al can be replaced by total Al (≥0.020%). Cold-rolled annealed condition: C≤0.20%, Mn≤1.00%
🏭 Melting Method (6.2) + Delivery Condition (6.3)
Steel shall be melted by converter or electric furnace, with ladle refining. Hot-rolled plates and strips shall be delivered in hot-rolled, controlled-rolled, or heat-treated condition; cold-rolled plates and strips shall be delivered in annealed condition.
⚙️ Mechanical Properties & Process Performance
| Grade | Lower Yield ReL (MPa) ≥ |
Tensile Rm (MPa) |
Elongation A80mm % ≥ |
Elongation A % ≥ |
180° Bend Test Bending Plunger Dia D |
|---|---|---|---|---|---|
| HP235 | 235 | 380~500 | 23 | 29 | D=1.5a |
📌 Both tensile and bend tests use transverse specimens; when yield phenomenon is not obvious, use Rp0.2; arbitration bend specimen width b=35mm.
❄️ Impact Properties
6.4.2 Room temperature Charpy impact test results shall be the average of three specimens, one specimen may be lower than the specified value but not less than 70% of the specified value.
6.4.3 For thickness >8~12mm, auxiliary specimen 10×7.5×55mm; for 6~8mm, auxiliary specimen 10×5×55mm; thickness <6mm no impact test.
| Grade | Specimen Size | RT Impact KV₂/J ≥ | -40℃ Impact KV₂/J ≥ (Optional) |
|---|---|---|---|
| HP235 | 10×5×55 or 10×7.5×55 | 23 | 18 |
🔹 Impact tests use transverse specimens. -40℃ impact can replace RT impact upon negotiation per 6.6 a).
🌾 Grain Size (6.5)
The grain size of steel plates and strips shall be not less than Grade 6, with grain size unevenness within three consecutive different grade levels. If the supplier can guarantee, inspection may be omitted.
📋 Additional Requirements (6.6)
Upon buyer's request and agreement between both parties, the following may be added:
a) -40℃ Charpy impact test, may replace room temperature impact test.
b) Yield ratio shall not exceed 0.80.
c) Non-metallic inclusions shall comply with Table 4:
| A | B | C | D | DS | Total |
|---|---|---|---|---|---|
| ≤2.0 | ≤2.0 | ≤2.0 | ≤2.0 | ＜2.0 | ≤7.0 |
✨ Surface Quality (6.7)
The surface of steel plates and strips shall have no cracks, scars, folds, blisters, inclusions, delaminations, or other harmful defects. Local pitting, depressions, scratches, and other minor defects with depth (height) not exceeding half of the thickness tolerance are permitted, provided the minimum thickness is guaranteed.
📏 Dimensions, Shape, Weight & Tolerances (Clause 5)
Hot-rolled plates and strips comply with GB/T 709 (thickness 2.0~14.0mm); cold-rolled plates and strips comply with GB/T 708 (thickness 1.5~4.0mm).
🧪 Inspection Rules (Clause 8)
8.1 Batching Rules: Each batch shall consist of steel plates or strips of the same grade, same heat number, same thickness, and same rolling or heat treatment system, with batch weight not exceeding 60 tons. Coils or continuously rolled plates with coil weight exceeding 30 tons may be batched as two rolling coils.
8.2.1 Impact Retest: If impact test results fail to comply, three additional specimens shall be taken from the same sampled product. The average of all six specimens shall not be less than the specified value, with up to two specimens allowed below the specified value, but only one specimen allowed below 70% of the specified value.
8.3 Rounding of Values: Chemical composition and mechanical property test results shall use the rounding value comparison method, with rounding rules according to GB/T 8170.
✅ Dedicated HP235 for Cylinders ✅ Weld Crack Resistance ✅ Low P/S Anti-brittle ✅ -40℃ Impact Toughness ✅ LF Refining + NDT ✅ Yield Ratio ≤0.80 Optional
🏭 Real Industrial Application: HP235 has been used for LPG cylinders, propane tanks, and industrial gas welded cylinders, validated by burst tests, pulse fatigue tests, and severe cold drop tests.
📢 For cylinder manufacturers exporting to North America, Europe, and Southeast Asia, HP235 meets equivalent requirements of ISO 4978 and EN 10120. With -40℃ impact data, it can smoothly pass TÜV, BV, and other third-party certifications.
⚙️ Standard Basis: GB/T 6653—2017 "Steel plates and strips for welded gas cylinders". This technical page fully references Chapter 3, 6.1~6.7, and Tables 1/2/3/4. All data is authentic and valid.
Data sourced from GB/T 6653-2017 Official Standard | Authoritative Welded Gas Cylinder Steel Data Platform