Pyrowear® 675 (UNS S42670) — AMS 5930 Carburizing Stainless Steel

1. Standard Overview

AMS 5930 governs the technical requirements for Pyrowear® 675 (UNS S42670), a carburizing-grade, corrosion-resistant martensitic stainless steel engineered specifically for high-demand wear and fatigue environments. The standard covers mill-produced solid forms — primarily bar-rounds and billets — intended for machining into precision components such as bearings, gears, and mechanical actuators.

Unlike conventional gear steels, Pyrowear® 675 was developed to address the simultaneous requirement for a hard, wear-resistant surface layer and a tough, ductile load-bearing core — two properties that traditional steels typically cannot deliver together. The alloy achieves this through a carefully balanced chemistry combining chromium for corrosion protection, cobalt and molybdenum for secondary hardening and elevated-temperature stability, vanadium for refined carbide dispersion, and nickel to suppress ferrite formation and strengthen the core.

The standard mandates specific heat treatment procedures — including a mandatory pre-oxidation step before carburizing — to ensure consistent case depth, surface carbon distribution, and final hardness. Dimensional, mechanical, and metallurgical requirements are all defined within AMS 5930, making it a comprehensive specification for aerospace and industrial procurement.

2. Grade Introduction

2.1 Core Characteristics

Pyrowear® 675 delivers a carburized case hardness exceeding HRC 60, combined with a core fracture toughness profile comparable to conventional case-hardening alloy steels widely used in gearing applications. The corrosion resistance of the carburized case is on par with high-chromium martensitic stainless grades, while the core maintains its own corrosion stability in domestic environments and mild industrial atmospheres.

A distinctive performance feature is the alloy’s hot hardness retention — case hardness remains above HRC 60 at temperatures up to approximately 315°C (600°F), making it well-suited for components operating under frictional heating or elevated ambient conditions.

2.2 Service Temperature Range

⚠️ Tempering in the range 371–482°C (700–900°F) is not recommended, as it causes a measurable reduction in both case and core toughness.

2.3 Available Product Forms and Size Range

Per AMS 5930, Pyrowear® 675 is produced in the following mill forms:

• Bar (Round): Solid rounds for machined components
• Billet: For forging and further processing

Typical size ranges for bar stock are from approximately 12 mm (0.5 in.) to 250 mm (10 in.) in diameter, though specific availability should be confirmed against current mill production schedules. Raw material is supplied in the annealed condition with a maximum hardness of Brinell 320 (target range: Brinell 280–320).

2.4 Typical Application Industries

3. Chemical Composition

The following composition is per AMS 5930 (type analysis; single figures are nominal unless noted as maximum).

Compositional Design Rationale:

• Chromium (13%) provides the baseline corrosion resistance and contributes to case carbide formation during carburizing.
• Cobalt (5.4%) raises the Ms temperature, stabilizes the matrix during secondary hardening, and is central to the alloy’s hot hardness capability.
• Molybdenum (1.8%) contributes to secondary hardening kinetics and improves resistance to contact fatigue (pitting) in cyclic loading environments.
• Vanadium (0.6%) refines the grain structure and forms fine carbides that improve wear resistance without compromising toughness.
• Nickel (2.6%) suppresses delta-ferrite formation and contributes to core ductility and low-temperature toughness.
• Low Carbon (≤ 0.07%) ensures the core remains in a ductile, low-hardness state prior to carburizing; all required surface carbon is introduced through the carburizing process itself.

4. Mechanical Properties

4.1 Core Tensile Properties (Typical)

All values represent typical properties on pseudocarburized test material following the standard heat treatment sequence: carburizing at 899°C → oil quench → anneal at 649°C → double normalize at 1038°C → anneal → austenitize at 1038°C / 15 min in salt → oil quench → refrigerate at -73°C / 1 hr → temper.

Tempered at 316°C (600°F) — 2 hrs + 2 hrs:

Tempered at 510°C (950°F) — 2 hrs + 2 hrs:

4.2 Core Fracture Toughness (Typical)

Note: The 316°C temper delivers significantly higher transverse toughness than the 510°C temper. Component orientation relative to bar-rolling direction should be factored into design selections.

4.3 Case Hardness vs. Tempering Temperature

The table below shows the effect of tempering temperature on carburized case and core hardness. Samples were austenitized at 1038°C / 30 min, oil quenched, refrigerated at -73°C / 1 hr, and tempered at the indicated temperature for 2 hrs + 2 hrs.

4.4 Annealed Condition Hardness

5. Physical Properties

5.1 Mean Coefficient of Thermal Expansion (CTE)

Annealed Condition:

Hardened and Tempered Condition:

6. Heat Treatment and Special Process Requirements

6.1 Annealing

Annealing is carried out at 621–649°C (1,150–1,200°F) for a minimum of 4 hours, followed by air cooling. This process is required before and after rough machining to restore machinability and relieve residual stress. Hardness after annealing should fall within Brinell 280–320.

6.2 Carburizing — Critical Pre-Oxidation Requirement

Pre-oxidation is mandatory before carburizing. Unlike standard case-hardening steels, the high chromium content of Pyrowear® 675 forms a passive oxide layer that inhibits carbon diffusion. Pre-oxidation breaks down this barrier by forming a controlled iron-oxide layer, enabling uniform carbon uptake during carburizing.

Pre-oxidation is performed by heating clean parts to 899–1038°C (1,650–1,900°F) for a minimum of 1 hour in an air atmosphere, followed by air cooling. Lower temperatures within this range (closer to 899°C) generally produce a better case microstructure.

Carburizing parameters:

Note: The standard carburizing temperature for many other grades (927°C / 1,700°F) is too high for this alloy and may produce coarse carbides in the case. Strict temperature control is essential.

Typical case depth (24-hour carburize at 871–899°C):

• HRC 60+ depth: ~0.020 in. (0.51 mm)
• HRC 50+ depth: ~0.045 in. (1.14 mm)

6.3 Hardening Sequence

Following carburizing and optional double-normalizing, the standard hardening cycle is:

1. Austenitize at 1,038°C (1,900°F) for 15–30 minutes (atmosphere-controlled furnace or salt pot)
2. Oil quench to room temperature (or salt quench at 204°C / 400°F, equalize, air cool to room temperature)
3. Refrigerate at -73°C (-100°F) for 1 hour; air warm to room temperature
4. Temper immediately — 2 hours + 2 hours at the selected temperature

The sub-zero refrigeration step is required to maximize conversion of retained austenite and achieve full case hardness.

6.4 Forging

Forging temperature range: 1,066–1,093°C (1,950–2,000°F). Forging should not continue below 927°C (1,700°F). Forgings should be annealed promptly after cooling.

6.5 Machining

Pyrowear® 675 in the annealed condition machines similarly to 410-series martensitic stainless steel. Recommended cutting parameters with cobalt high-speed steel tooling:

Parts intended for carburizing should be thoroughly degreased prior to the heat treatment sequence to ensure uniform carbon uptake.

7. Corrosion Resistance

The carburized case of Pyrowear® 675 exhibits corrosion resistance broadly equivalent to that of high-chromium martensitic stainless grades in the hardened condition. The core exhibits corrosion resistance comparable to standard 12% chromium martensitic grades, which is generally superior to the case in aggressive media.

Humidity resistance: Polished samples (both core and carburized case) show no visible rust after 200 hours at 35°C / 95% relative humidity.

Corrosion media summary:

Corrosion behavior depends heavily on surface condition. Post-machining cleaning and passivation are recommended for optimal field performance. Actual suitability for specific corrosive environments should be verified through application-specific testing.

8. Inspection and Testing Requirements

Procurement and acceptance testing under AMS 5930 typically includes the following:

Chemical Analysis:

• Heat analysis and product (check) analysis to confirm compliance with the compositional requirements above.
• All elements are subject to verification; cobalt, chromium, and molybdenum are particularly critical to final performance.

Mechanical Testing:

• Tensile testing of heat-treated specimens in both longitudinal and transverse orientations.
• Hardness verification in both annealed supply condition and after representative heat treatment.
• Charpy V-notch impact testing may be specified for fracture-critical aerospace applications.

Nondestructive Evaluation:

• Ultrasonic inspection of bar and billet material to detect internal discontinuities; acceptance criteria per AMS 2631 or customer-specific drawing requirements.
• Magnetic particle inspection (MT) may be applied to finished machined surfaces per AMS 2301 for surface and near-surface discontinuity detection.
• Dimensional verification per applicable drawing callouts.

Microstructural Requirements:

• Case microstructure assessed for carbide distribution and uniformity.
• Core grain size verification per applicable heat treatment certification.

9. About C&N | Hongcheng Pipe Fittings Limited

At C&N (Hongcheng Pipe Fittings Limited), we work with specialty alloys because we understand that precision components in demanding industries cannot afford material uncertainty. Pyrowear® 675 / UNS S42670 is one of the grades we supply with full traceability — mill certifications, chemical and mechanical test reports, and heat treatment records available as standard.

Whether you’re sourcing bar stock for aerospace bearing machining, evaluating material for a high-load gearing application, or building a qualification package for an OEM approval, our team can support the process — from grade selection through to documentation.

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