Background: Two Grades That Look Almost Identical
- Si 0.17–0.37% (residual element only)
- Cr: 0.70–1.15%, Mo: 0.40–0.60%
- Reliable up to ~430°C continuous service
- ASTM near-equivalent: A387 Gr.11 / A182 F11
- Lower cost; widely stocked globally
- Si 0.50–0.80% (intentional alloying element)
- Cr: 1.00–1.50%, Mo: 0.45–0.65%
- Reliable up to ~520°C continuous service
- No direct ASTM equivalent
- Required for EN code-governed service above 460°C
When a procurement engineer first encounters EN 1.7336 (13CrMoSi5-5) and EN 1.7335 (13CrMo4-5) side by side, the immediate reaction is often: these must be the same thing. Both are low-alloy chromium-molybdenum heat-resistant steels governed by EN 10028-2:2009. Both share nearly identical Cr and Mo ranges. Both are delivered normalized and tempered. Both serve pressure equipment in the 400–520°C band.
This surface similarity is precisely where the engineering risk lies. In our 21+ years of manufacturing experience at Jiangsu Liangyi Co., Limited, we have seen purchase orders specifying 1.7336 arrive with a vendor quote offering 1.7335 at a lower price — sometimes with the note "equivalent grade, no performance difference." This is technically incorrect, and in many code-governed applications, it is also contractually and legally non-compliant.
This analysis applies to forged components (bars, rings, flanges, valve bodies, pressure vessel shells) governed by EN 10028-2, EN 10222-2, EN 13480, and EN 12952. ASME/ASTM equivalents are referenced for cross-standard clarity but are not the primary scope.
Chemical Composition: Where Silicon Changes Everything
The most frequently overlooked difference between the two grades is silicon content. EN 10028-2:2009 specifies the following compositional limits (heat analysis, mass fraction %):
| Element | 1.7335 — 13CrMo4-5 | 1.7336 — 13CrMoSi5-5 | Significance |
|---|---|---|---|
| C (Carbon) | max 0.17% | max 0.17% | Same; carbide-forming base balanced for weldability |
| Si (Silicon) | 0.17–0.37% | 0.50–0.80% | Key difference — elevated Si is the defining feature of 1.7336 |
| Mn (Manganese) | 0.40–0.65% | 0.40–0.65% | Near-identical; deoxidation and hardenability |
| Cr (Chromium) | 0.70–1.15% | 1.00–1.50% | 1.7336 slightly higher; improved oxidation resistance |
| Mo (Molybdenum) | 0.40–0.60% | 0.45–0.65% | Near-identical; solid-solution creep strengthener |
| P (Phosphorus) | max 0.025% | max 0.015% | 1.7336 tighter; grain boundary embrittlement control |
| S (Sulfur) | max 0.010% | max 0.005% | 1.7336 tighter; VD process required for inclusion cleanliness |
The grade designation encodes the silicon addition: the suffix Si5-5 in 13CrMoSi5-5 denotes silicon as an intentional alloying element. In 13CrMo4-5, no silicon suffix appears because silicon is a residual element. EN 10028-2 also sets tighter P and S limits for 1.7336 — reflecting that it is designed for more demanding service and requires cleaner steelmaking through a VD (vacuum degassing) process route.
Many engineers assume the "5-5" suffix in both grades refers to the same elements. In 13CrMo4-5, "4" approximates Cr×4; "5" approximates Mo×10. In 13CrMoSi5-5, "Si5" refers to Si×5; the final "5" still refers to Mo×10. This naming overlap is a persistent source of procurement errors.
The Allowable Stress Divergence Above 480°C
The most operationally critical distinction lies in the time-dependent allowable stress tables published in EN 10028-2:2009 Annex B — the tables that define maximum permissible stresses used in pressure vessel and piping design calculations under EN 13480 and EN 12952. Below 430°C, the two grades are essentially equivalent. Above 480°C, a significant and engineering-critical divergence appears.
| Temperature | 1.7335 approx. stress | 1.7336 approx. stress | 1.7336 advantage |
|---|---|---|---|
| 350°C | ~175 MPa | ~177 MPa | ≈ Equivalent ✓ |
| 400°C | ~162 MPa | ~165 MPa | ≈ Equivalent ✓ |
| 430°C | ~148 MPa | ~152 MPa | +3% — borderline ✓ |
| 460°C | ~122 MPa | ~138 MPa | +13% — significant ⚠ |
| 480°C | ~98 MPa | ~122 MPa | +24% — critical ✗ |
| 500°C | ~72 MPa | ~96 MPa | +33% — non-substitutable ✗ |
| 520°C | ~51 MPa | ~74 MPa | +45% — non-substitutable ✗ |
"If a component was designed to operate at 500°C using 1.7336 allowable stresses and a supplier delivers 1.7335 instead, the material cannot sustain the design loads. The component is under-designed by up to one-third of its required creep strength."
— Jiangsu Liangyi Co., Limited Engineering Team · Jiangyin, Jiangsu, ChinaWhy Silicon Raises the Creep Threshold
Silicon's role in 13CrMoSi5-5 operates through three distinct metallurgical mechanisms that collectively delay creep deformation onset above 460°C.
1. Solid-solution strengthening of the ferrite matrix
Silicon dissolves substitutionally in the ferritic matrix. At elevated temperatures, Si atoms create local lattice distortions that impede dislocation climb and glide — the primary microscale mechanisms in creep deformation. This effect scales with Si concentration, which is why the 0.50–0.80% Si range in 1.7336 creates a measurable strengthening increment that 1.7335's Si content (0.17–0.37%) cannot replicate.
2. Oxidation barrier formation (SiO₂ subscale)
Silicon promotes the formation of a thin, adherent SiO₂-enriched oxide subscale beneath the primary Cr₂O₃ surface oxide layer. This subscale significantly reduces the rate of high-temperature scaling at 480–520°C in steam or oxidizing combustion gas atmospheres. Published industry data on 1.25Cr-0.5Mo steel families indicate that higher silicon content in the 0.5–0.8% range produces measurably lower oxidation scaling rates at 550°C compared to equivalent steels with lower silicon — due to the formation of this protective SiO₂ sublayer beneath the outer Cr₂O₃ scale. In our own internal accelerated oxidation testing this advantage is measurable and consistent with the published literature, and is a key reason major European boiler manufacturers prefer 1.7336 over 1.7335 for steam header and valve forgings operating above 480°C.
3. Carbide coarsening retardation
Long-term creep strength in CrMo steels depends on a fine, stable distribution of M₂C and M₇C₃/M₂₃C₆ carbides along lath boundaries. Above 460°C, these carbides progressively coarsen (Ostwald ripening), reducing obstacle density for dislocation movement. Silicon modifies the activity coefficients of Mo and Cr in the matrix, slowing this coarsening kinetics — maintaining creep resistance at higher temperatures for longer service durations than 1.7335.
In our production of thick-section 1.7336 forgings (diameter above 400 mm), we monitor carbide morphology on sectioned test coupons after heat treatment. The carbide spacing in normalized-and-tempered 1.7336 consistently measures finer than equivalent-section 1.7335 processed under the same thermal cycle — a microstructural difference directly reflected in the Larson-Miller creep-rupture data that underpins the EN 10028-2 allowable stress tables. The tighter P and S limits and VD steelmaking requirement for 1.7336 also contribute to consistent, predictable properties across thick sections.
EN 13480 and EN 12952: What the Standards Actually Say
The legal and contractual prohibition on grade substitution above 480°C stems from how pressure equipment codes reference allowable stress values. Both EN 13480 (metallic industrial piping) and EN 12952 (water-tube boilers) require that:
- Design calculations use the permissible stress values published for the specific material grade stated on the material certificate.
- Any change in material grade must be accompanied by re-verification of all affected design calculations.
- The material grade on the EN 10204 Type 3.1 or 3.2 Mill Test Certificate must match the grade specified on the purchase order and design drawings.
Since EN 10028-2 publishes different permissible stress values for 1.7336 and 1.7335 above 480°C, substituting one for the other without new design calculations constitutes a non-conformance under EN 13480 §6 and EN 12952 §6. In CE-marked pressure equipment, this invalidates the conformity assessment under the Pressure Equipment Directive (PED) 2014/68/EU.
Delivering 1.7335 with a certificate declaring 1.7336 constitutes material misrepresentation under applicable commercial law and PED Article 48. Openly proposing 1.7335 as a "grade equivalent" on a code-governed project above 480°C requires formal deviation approval from the design responsible engineer, the end user, and potentially the notified body. In practice, this approval is rarely granted for the 480–520°C service range. Always obtain the current edition of EN 10028-2 directly from CEN or your national standards body and verify the grade-specific stress values with your design code before accepting any material substitution.
Temper Embrittlement: A Compounding Risk in Both Grades
Both 1.7335 and 1.7336 are susceptible to temper embrittlement — a reduction in room-temperature toughness (Charpy impact energy) that develops after long-term exposure in the 350–575°C range, involving segregation of phosphorus, antimony, tin, and arsenic to prior-austenite grain boundaries. The embrittlement does not reduce high-temperature creep strength, but creates serious risk during plant startups and shutdowns — a DBTT shift of +50 to +100°C after 20+ years of service is not uncommon.
Importantly, the higher silicon content in 1.7336 (0.50–0.80%) elevates the J-factor (a standard temper embrittlement susceptibility index) compared to 1.7335 for the same P and Sn levels. This is why applications above 460°C using 1.7336 routinely require vacuum degassing steelmaking — a process step Jiangsu Liangyi specifies as standard for all 1.7336 billet supply, achieving P ≤ 0.012% and S ≤ 0.004% in our production chemistry (tighter than the EN 10028-2 minimums of P ≤ 0.015%, S ≤ 0.005%).
| Control parameter | Typical specification | Purpose |
|---|---|---|
| VD steelmaking | P ≤ 0.015%, S ≤ 0.005% (EN minimum); Jiangsu Liangyi aim: P ≤ 0.012%, S ≤ 0.004% | Removes primary grain-boundary segregants |
| Tramp element control | Sb + Sn + As ≤ 0.025% (project-specific; state in purchase order) | Limits long-term segregation kinetics |
| J-factor limit | J = (Mn+Si)(P+Sn)×10⁴ ≤ 100 (specify in purchase order if required) | Temper embrittlement susceptibility index |
| X-factor limit | X = (10P+5Sb+4Sn+As)/100 (specify maximum if required for nuclear or high-integrity applications) | Used for reactor pressure vessel code compliance |
When Is 1.7335 Acceptable? Correct Substitution Scenarios
EN 1.7335 is an excellent, well-proven grade with a long service record. The question is always one of application fit, not grade quality.
| Service condition | Correct grade | Reason |
|---|---|---|
| Continuous service ≤ 430°C | Either grade | EN 10028-2 allowable stresses essentially equivalent; 1.7335 may offer lower cost |
| Service 430–480°C, non-code application, with documented stress re-verification | 1.7335 conditionally | Only if wall thickness and pressure ratings are formally recalculated using 1.7335 EN 10028-2 stress values and the responsible engineer signs off |
| Continuous service 480–520°C | 1.7336 only | 1.7335 EN 10028-2 stress values are insufficient for components designed to 1.7336 allowable stresses. See our full range of 1.7336 forged bars, rings and hollow forgings for available sizes and custom options. |
| EN 13480 / EN 12952 code-governed at T above 460°C | 1.7336 only | Grade substitution requires formal re-verification and deviation approval that is rarely practical to obtain |
| Original design specifies 1.7336 | 1.7336 only | The MTC must match the design specification; deviation requires formal concession approval |
| Replacement or repair parts for existing 1.7336 equipment | 1.7336 only | Like-for-like replacement is mandatory under most pressure equipment inspection codes |
The reverse substitution — supplying 1.7336 where 1.7335 was specified — is almost always permitted without re-verification of allowable stresses, because 1.7336 has equal or higher permissible stresses at all temperatures. The only consideration is the 5–10% cost premium of 1.7336 over 1.7335 at equivalent forging sizes.
Procurement Checklist: Avoiding a Non-Compliant Supply
Based on 21+ years of supplying CrMo forgings to global pressure equipment projects, these six checkpoints protect buyers from receiving non-compliant or misrepresented material.
Write "EN 1.7336 (13CrMoSi5-5) per EN 10028-2:2009" on the purchase order. Not "CrMo 1Cr 0.5Mo" or "13CrMo." The numerical designation is the legally binding specification reference.
The MTC must show Si in the 0.50–0.80% range per EN 10028-2 for 1.7336. If Si is below 0.40%, the material may not meet 1.7336 specification regardless of what the grade designation line states.
For forgings above 150 mm section, request product analysis cut from the actual forging in addition to the heat analysis. Silicon can stratify in large ingots; product analysis confirms actual composition in the finished section.
Request the VD steelmaking record from the billet producer. If temper embrittlement susceptibility is critical for your application, specify a maximum J-factor (e.g. J ≤ 100) explicitly on the purchase order.
If a supplier proposes 1.7335 as an alternative to 1.7336, require a signed engineering document showing: (a) revised allowable stress calculations at design temperature using 1.7335 EN 10028-2 values, and (b) written confirmation that the resulting wall thickness and pressure rating still meet design requirements.
For service above 480°C, you may require your nominated independent inspection body to witness production testing and verify material identity before despatch. Jiangsu Liangyi supports EN 10204 Type 3.2 witness inspection by client-nominated bodies — please state this requirement at the enquiry stage so we can confirm feasibility and adjust the inspection schedule accordingly. Full details of our 1.7336 manufacturing and inspection capabilities — including NDT scope, certifications, and inspection body support — are documented on our product page.
Decision Framework: Which Grade Do You Need?
Use this structured decision path for any CrMo pressure component project requiring a 1.0–1.5Cr, 0.5Mo alloy steel under EN standards:
Below 430°C → either grade acceptable (go to Q3). 430–480°C → go to Q2. Above 480°C → specify 1.7336. Stop here.
If yes → specify 1.7336 to eliminate re-verification burden. If no → 1.7335 conditionally acceptable with formal allowable stress recalculation and responsible engineer sign-off.
If 1.7336 is specified → supply 1.7336 only. If 1.7335 is specified and temperature is below 430°C → supply 1.7335. Upgrading to 1.7336 for performance margin is always permitted without re-verification.
Even below 430°C, if the component will operate in a steam or oxidizing atmosphere for a multi-decade design life, 1.7336's superior oxidation resistance from its SiO₂ subscale-forming capability may justify the modest cost premium on a life-cycle cost basis.