SPA-H vs SPA-C – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers and procurement teams selecting pressure‑vessel or structural plate steels often confront a tradeoff between strength, toughness, and cost. Typical decision contexts include specifying plates for boilers and pressure vessels, choosing materials for cold‑service tanks, or selecting heavier sections where thinner gauges are desirable to save weight. SPA‑C and SPA‑H are compared frequently because they represent two different design philosophies: one emphasizes lower carbon and higher as‑fabricated toughness and weldability, while the other emphasizes greater hardenability and higher achievable strength via composition and heat treatment.

The practical difference between these grades centers on alloying and carbon/hardenability strategy: SPA‑C formulations are optimized for ductility, notch toughness and good weldability at the expense of maximum strength, whereas SPA‑H formulations contain higher hardenability and alloy content to enable higher strength and/or better strength retention at elevated temperatures but may require more exact heat‑treatment and welding controls.

1. Standards and Designations

• Common standards where SPA‑type nomenclature appears: ASME/ASTM material lists and pressure‑vessel plate catalogs; however, exact usage varies by supplier and region. Always confirm the precise specification and certificate (e.g., ASTM Axxx or EN xxxx) before procurement.
• Other standards to consider when comparing similar steels: EN (European norms), JIS (Japanese), GB (Chinese national standards).
• Classification by steel family:
• SPA‑C: Typically a carbon or low‑alloy carbon steel designed for pressure‑vessel plate service (carbon steel family).
• SPA‑H: Typically a higher‑hardenability or low‑alloy steel (still often categorized as carbon/alloy steel rather than stainless or tool steel) intended for higher strength applications (low‑alloy/carbon steel family).
• Note: SPA‑prefix nomenclature is sometimes used in supplier catalogs or older material lists; the underlying standardized specification (ASTM/EN/JIS/GB) dictates exact chemistry and mechanical requirements.

2. Chemical Composition and Alloying Strategy

The table below gives indicative, typical composition ranges (wt%). These are representative ranges used in engineering practice to illustrate the compositional strategy — exact values must be taken from the controlling specification or mill certificate.

Element	Typical SPA‑C (wt%) — indicative	Typical SPA‑H (wt%) — indicative
C	0.06 – 0.20	0.15 – 0.35
Mn	0.3 – 0.9	0.5 – 1.2
Si	0.10 – 0.40	0.10 – 0.50
P	≤ 0.025 – 0.035	≤ 0.030 – 0.040
S	≤ 0.025 – 0.035	≤ 0.030 – 0.040
Cr	≤ 0.30	0.20 – 1.00
Ni	≤ 0.30	0.20 – 1.50
Mo	≤ 0.10	0.05 – 0.60
V	≤ 0.05	0.02 – 0.20
Nb (Cb)	trace – 0.02	trace – 0.06
Ti	trace – 0.02	trace – 0.05
B	trace (often none)	trace (ppm levels when used)
N	≤ 0.012	≤ 0.012

How alloying affects properties - Carbon: primary hardenability and strength control; higher carbon increases achievable strength and hardness but reduces ductility and weldability. - Manganese: increases hardenability, tensile strength, and deoxidation; high Mn aids strength but may slightly reduce toughness if excessive. - Silicon: deoxidizer, small solid solution strengthening. - Chromium, Molybdenum, Nickel, Vanadium, Niobium, Titanium: alloying additions that increase hardenability, strength after quench/temper, and high‑temperature strength (Mo, Cr). Microalloying (V, Nb, Ti) refines grain and improves strength/toughness balance via precipitation strengthening and grain‑refinement. - Boron (ppm): very small additions can dramatically increase hardenability when present at controlled levels.

3. Microstructure and Heat Treatment Response

Typical microstructures - SPA‑C: As‑rolled or normalized condition usually exhibits ferrite‑pearlite (or fine ferrite/pearlite) microstructure. Low carbon and limited alloying yield coarse or fine pearlite depending on cooling history; toughness is achieved by limiting carbon and controlling cleanliness and grain size. - SPA‑H: With higher carbon and alloy content, SPA‑H steels can develop bainitic or martensitic structures after suitable quenching or controlled cooling. In normalized or quenched‑and‑tempered conditions, they show tempered martensite or tempered bainite with higher strength.

Heat treatment response - Normalizing: both grades respond to normalizing with refined grain size and improved toughness; SPA‑C benefits more readily due to lower hardenability. - Quench & temper: SPA‑H is designed to be quenched and tempered to achieve high strength and controlled toughness; SPA‑C may be less commonly quenched into martensite because of lower carbon and less alloying (hardening response is limited). - Thermo‑mechanical processing: microalloyed variants of both grades (with Nb, V) respond well to controlled rolling to produce fine‑grained ferrite–pearlite or bainitic structures that offer an improved strength–toughness balance.

4. Mechanical Properties

Typical mechanical property ranges — indicative only; use the controlling specification for procurement.

Property	SPA‑C (typical range)	SPA‑H (typical range)
Tensile strength (MPa)	380 – 550	500 – 900
Yield strength (0.2% offset) (MPa)	230 – 350	350 – 700
Elongation (%)	18 – 30	8 – 20
Impact toughness (Charpy V‑notch)	Often ≥ 27 J at specified temp; good low‑temperature toughness	Variable; can be good if tempered correctly but generally lower than SPA‑C in as‑delivered state
Hardness (HB)	~120 – 200	~160 – 320

Interpretation - SPA‑H can reach higher strength levels due to higher carbon and alloying plus heat treatment, but this often reduces ductility and notch toughness compared with SPA‑C. - SPA‑C is typically more ductile and has better as‑fabricated notch toughness and weldability, suitable for cold or impact‑sensitive environments.

5. Weldability

Weldability depends on carbon equivalent and hardenability rather than name alone. Two commonly used empirical indices are the IIW carbon equivalent and the International Welding Institute’s Pcm. Examples:

$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation - SPA‑C: lower carbon and fewer hardenability alloys yield lower $CE_{IIW}$ and $P_{cm}$; this translates to better weldability, lower preheat requirements, and lower risk of hydrogen‑induced cracking if proper welding practices are used. - SPA‑H: higher carbon and alloy content increase $CE_{IIW}$/$P_{cm}$, raising the risk of hardened heat‑affected zones and cold cracking. SPA‑H may require preheat, controlled interpass temperatures, post‑weld heat treatment (PWHT), and low‑hydrogen consumables. - Microalloying: elements like Nb and V can slightly raise $P_{cm}$ while improving grain size and strength; their effect on weldability must be managed via welding procedure specification (WPS).

6. Corrosion and Surface Protection

• Both SPA‑C and SPA‑H are non‑stainless carbon/low‑alloy steels; uniform corrosion protection relies on coatings and cathodic protection.
• Common protective measures: hot‑dip galvanizing (suitable for many carbon steels but consider temperature limits and thickness), epoxy/urethane paint systems, metallizing, and sacrificial anodes for immersed service.
• For environments with high chloride content or where passivity is required, stainless steels are required; PREN (pitting resistance equivalent number) is not applicable to carbon steels. For reference, PREN is:

$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

• Protective strategy selection depends on service (atmospheric, immersed, sour service), temperature, and allowable maintenance.

7. Fabrication, Machinability, and Formability

• Cutting: SPA‑H's higher hardness and strength increase tool wear and may require slower cutting speeds and tougher tooling. SPA‑C machines more easily.
• Forming/bending: SPA‑C is more formable due to lower yield strength and higher ductility; SPA‑H may require larger bend radii or heat‑assisted forming.
• Welding/fabrication: SPA‑C generally tolerates more aggressive fabrication practices; SPA‑H requires controlled preheat, interpass, and possibly PWHT for critical fabrication. Residual stress and distortion management are more important with SPA‑H because of higher strength gradients.
• Surface finishing: Both respond to grinding, shot blasting, and surface treatments, but SPA‑H may exhibit work‑hardening or tempering effects if subjected to high‑energy finishing.

8. Typical Applications

SPA‑C — Typical uses	SPA‑H — Typical uses
Boiler and pressure vessel shells where high notch toughness and good weldability are essential (low‑temp service).	Pressure boundary components and structural parts where higher strength or thinner sections are desired (when design requires higher allowable stresses).
Storage tanks and piping for moderate pressure and ambient/lower temperatures.	High‑pressure vessels, thicker cross‑sections that require deep hardening, and quenched‑and‑tempered components.
General structural plate where impact toughness and ductility are prioritized.	Heavy machinery components, forged parts, and applications requiring high tempering resistance.

Selection rationale - Choose SPA‑C when low‑temperature toughness, in‑service impact resistance, and straightforward welding are priorities. - Choose SPA‑H when the design needs higher allowable stresses, thinner sections for the same load, or when the component will be quenched and tempered to target a specific strength.

9. Cost and Availability

• Cost: SPA‑H typically carries a premium over SPA‑C due to higher alloy content, additional processing (controlled rolling, quench/temper), and tighter heat treatment controls. SPA‑C is generally more economical for large area plates where high toughness and weldability suffice.
• Availability: Both grades are commonly available in plate forms, but SPA‑C variants are more widespread in standard pressure‑vessel plate inventories. SPA‑H may be produced to order in specific thicknesses and condition (normalized, quenched & tempered), so lead times can be longer for unusual sizes or certified heat treatments.
• Product forms: plate, coil, and occasionally bars; SPA‑H is more often specified for heat‑treated plate and forgings.

10. Summary and Recommendation

Summary table (qualitative)

Attribute	SPA‑C	SPA‑H
Weldability	High (lower preheat, simpler WPS)	Moderate to lower (likely preheat/PWHT)
Strength–Toughness balance	Good toughness, moderate strength	High strength achievable, toughness depends on treatment
Cost	Lower	Higher

Recommendations - Choose SPA‑C if: - Your design requires superior as‑fabricated toughness and the lowest practical welding complexity. - The operating temperature is low or impact resistance is a key failure‑mode control. - Cost and ease of fabrication are dominant requirements.

• Choose SPA‑H if:
• You need higher allowable stresses, thinner sections for weight or space savings, or parts that will be quenched and tempered for specific strength levels.
• You can accept stricter welding controls (preheat, PWHT) and potentially higher procurement and processing costs.

Final note: SPA‑style labels can encompass a range of chemistries and conditions across suppliers. Always specify the controlling standard or mill test certificate, required heat treatment (normalized, quenched & tempered, or as‑rolled), Charpy‑V requirements at the governing temperature, and welding procedure qualifications. For critical designs, request full composition and mechanical test results, and perform prequalification welds and PWHT trials to validate performance in the intended fabrication and service environment.