Grade ST35 Seamless Carbon Steel Pipe
Overview
ST35 is a historical German material grade designation for carbon steel pipes and tubes. It refers to material with minimum tensile strength of approximately 35 kg/mm² (≈ 343 MPa). The designation is obsolete and has been superseded by modern European (EN) standards, but may still appear in legacy systems and documentation.
Important Clarification: ST35 is NOT a current standard. It was part of the old DIN 1629 and DIN 17175 standards which have been replaced by EN 10216 series.
Historical Grades & Modern Equivalents
| Old DIN Designation | Equivalent EN 10216-2 Grade | Key Characteristics |
|---|---|---|
| ST35.0 | P195TR2 | Basic carbon steel, room temperature service |
| ST35.4 | P235TR2 | Carbon steel for elevated temperatures (to 300°C) |
| ST35.8 | P235GH | Carbon steel for high temperature service (to 350°C) |
| ST35.8/III | P235GH + additional testing | Higher quality with impact testing |
Nomenclature Decoded:
ST = Steel ("Stahl" in German)
35 = Approximate tensile strength in kg/mm²
.0/.4/.8 = Quality/application level (8 = high temperature)
/III = Additional quality/testing level
Typical Properties (Historical Reference)
| Property | ST35.4 | ST35.8 | Notes |
|---|---|---|---|
| Tensile Strength | 360-480 MPa | 360-510 MPa | Range specified, not minimum |
| Yield Strength | ≥ 235 MPa | ≥ 235 MPa | Minimum value |
| Elongation | ≥ 25% | ≥ 25% | Minimum |
| Maximum Temperature | 300°C | 350°C | For pressure service |
| Impact Test | Not required | Optional | For ST35.8/III only |
Chemical Composition (Historical, Maximum %)
| Element | ST35.4 | ST35.8 |
|---|---|---|
| Carbon (C) | 0.17 | 0.17 |
| Silicon (Si) | 0.35 | 0.35 |
| Manganese (Mn) | 0.80 | 0.80 |
| Phosphorus (P) | 0.035 | 0.035 |
| Sulfur (S) | 0.035 | 0.035 |
| Aluminum (Altot) | ≥ 0.020 | ≥ 0.020 |
All ST35 grades were fine grain killed steel with aluminum addition for grain refinement.
Manufacturing Process (Historical)
Production for Seamless ST35:
Steel Making: Open hearth or basic oxygen furnace
Deoxidation: Aluminum killed for fine grain structure
Hot Working: Mannesmann plug mill process
Heat Treatment:
ST35.0/ST35.4: Often as-rolled
ST35.8: Normalized or normalized & tempered
Testing: Basic mechanical and hydrostatic tests
Size Range (Typical):
Outside Diameter: 21.3 mm to 406.4 mm
Wall Thickness: 2.0 mm to 40.0 mm
Length: 6-12 meters
Modern Replacement: EN 10216-2 P235GH
Why P235GH Replaces ST35.8:
| Aspect | ST35.8 (Obsolete) | P235GH (Current) |
|---|---|---|
| Standard Status | Withdrawn DIN standard | Current EN harmonized standard |
| Yield Strength | ≥ 235 MPa | ≥ 235 MPa |
| Temperature Rating | Up to 350°C | Up to 350°C (guaranteed) |
| Chemical Control | Basic controls | Stricter (S ≤ 0.020%, P ≤ 0.025%) |
| Traceability | Limited | Full EN 10204 3.2 certification |
| CE Marking | Not possible | Required for PED compliance |
P235GH Key Specifications:
Minimum Yield: 235 MPa
Tensile Range: 360-510 MPa
Carbon Max: 0.16% (vs 0.17% for ST35)
Sulfur Max: 0.020% (vs 0.035% for ST35)
Aluminum: ≥ 0.020% (fine grain killed)
Heat Treatment: Normalized or normalized & tempered
Applications (Historical & Modern)
Traditional Uses of ST35:
German Industrial Plants: Pre-2000 installations
Power Generation: Moderate temperature steam systems
Heating Systems: High-temperature hot water
Industrial Gas: Compressed air and gas lines
Chemical Industry: Process piping in older facilities
Modern Applications of P235GH:
Boiler Tubes: For temperatures up to 350°C
Heat Exchangers: Shell and tube
Process Piping: Steam, hot water, thermal fluids
Power Plants: Auxiliary steam systems
Industrial: General high-temperature service
Ordering Modern Equivalent
For New Projects/Replacements:
Specify: EN 10216-2 P235GH Seamless Pipe
Sample Order Specification:
text
Material: Seamless steel tube to EN 10216-2 Grade: P235GH Heat treatment: Normalized (+N) Dimensions: [OD in mm] × [wall thickness in mm] × [length in mm] Quantity: [number of pieces or meters] End preparation: [plain, bevelled, etc.] Certification: EN 10204 3.2 with full traceability Additional requirements: [if any special testing needed] Marking: Per EN 10216-2 including heat number
Example Conversion:
If old drawings specify: "ST35.8 seamless pipe, 60.3 × 4.5 × 8000 mm"
Replace with: "EN 10216-2 P235GH seamless pipe, 60.3 × 4.5 × 8000 mm, normalized"
Testing & Inspection Comparison
| Test | ST35 (Historical) | P235GH (Modern) |
|---|---|---|
| Hydrostatic Test | Basic pressure test | Calculated pressure, min 10 sec duration |
| Tensile Test | Room temperature only | Room temperature + optional elevated temp |
| Impact Test | Optional for ST35.8/III | Optional (not mandatory for P235GH) |
| Hardness Test | Optional | Optional |
| NDE | Not required | Optional (UT, RT by agreement) |
| Documentation | Basic certificate | EN 10204 3.2 with full traceability |
Why ST35 is Obsolete
Technical Reasons:
Inconsistent Standards: Multiple ST35 variants with unclear differences
Limited Testing: Basic requirements compared to modern standards
Poor Traceability: Limited material history documentation
Chemical Controls: Looser than modern requirements
Regulatory Reasons:
Not PED Compliant: Cannot be CE marked for pressure equipment
Withdrawn Standard: No longer maintained by standards body
Limited Acceptance: Not accepted in most new projects
Quality System Issues: Doesn't meet modern QA requirements
Procurement Considerations
If You Must Source "ST35":
Verify Exact Designation: ST35.0, ST35.4, or ST35.8?
Consider Modern Equivalent: P235GH is readily available
Check Project Requirements: May mandate modern standards
Evaluate Cost: "ST35" may cost more due to special production
Typical Scenarios:
| Scenario | Recommended Action |
|---|---|
| New project design | Use P235GH (EN 10216-2) |
| Replacement in existing plant | Use P235GH if acceptable to inspector |
| Legacy equipment spare | Source to original spec if absolutely necessary |
| Drawing says "ST35" | Update drawing to P235GH |
Industry Practice Today
European Market:
No mills produce to ST35 specification for new production
P235GH is the standard material for equivalent applications
All new projects specify EN standards, not obsolete DIN
Global Projects:
European engineering companies use EN 10216-2
International EPC contractors accept P235GH as equivalent
ASTM A106 Gr. B may also be acceptable alternative
Material Certification
For ST35 (Historical):
DIN 50049 3.1.B Certificate: Basic declaration
Limited Information: Chemical analysis, tensile test
Language: Typically German
For P235GH (Modern):
EN 10204 3.2 Certificate: Specific inspection certificate
Full Traceability: Complete material history
Comprehensive Testing: All required tests documented
International Language: Typically English available
Fabrication & Welding
Welding ST35/P235GH:
Preheat: Generally not required for thin sections
Filler Metal: AWS ER70S-6 or equivalent suitable
Post-Weld Heat Treatment: Optional for stress relief
Procedure Qualification: Per EN ISO 15614-1 or equivalent
Key Consideration:
ST35 and P235GH have similar weldability due to comparable carbon equivalents and fine grain structure.
Cost Comparison
Typical Price Relationships:
P235GH (standard production): 100% (baseline)
"ST35" (special production): 120-150% (if available)
ASTM A106 Gr. B: 90-110%
API 5L Gr. B: 95-105%
Factors Increasing Cost for ST35:
Special Production Run: Non-standard specification
Limited Suppliers: Few mills willing to produce
Additional Testing: May require verification testing
Documentation: Translation/verification of old certificates
Quality Assurance Issues
Challenges with ST35:
Verification Difficult: Hard to verify compliance with withdrawn standard
Property Variations: Different mills may have interpreted differently
Aging Documentation: Old certificates may be incomplete
Inspection Uncertainty: Inspectors may reject obsolete material
Benefits of P235GH:
Clear Specification: Well-defined requirements
Standard Production: Mills produce regularly
Full Documentation: Modern certificate format
Inspection Acceptance: Widely accepted by inspectors
Summary & Recommendations
Technical Summary:
ST35 is an obsolete German carbon steel pipe grade that has been completely replaced by EN 10216-2 P235GH. The modern equivalent offers:
Identical mechanical properties (235 MPa min yield)
Superior chemical controls (lower sulfur, phosphorus)
Modern certification and traceability
PED compliance for CE marking
Global availability and acceptance
Key Recommendations:
For All New Projects:
Specify EN 10216-2 P235GH
Never specify "ST35" in new designs
For Existing Plant Maintenance:
Use P235GH as replacement for ST35
Obtain engineering approval for material substitution
Update maintenance databases to reflect modern equivalent
When Encountering "ST35" in Documentation:
Determine exact grade (ST35.0, .4, .8)
Consult with materials engineer on replacement
Update documentation to modern standard
Procurement Strategy:
Source P235GH from reputable mills
Require EN 10204 3.2 certification
Verify material meets project requirements
Final Note:
While ST35 served industry well for decades, it has no place in modern engineering. The transition to EN 10216-2 P235GH provides better quality assurance, regulatory compliance, and long-term reliability for high-temperature pressure applications.
*For applications requiring different properties (higher strength, different temperature ranges), other EN 10216-2 grades (P265GH, 16Mo3, etc.) should be considered instead of trying to source obsolete ST35 variants.*
