Introduction
When temperatures climb toward 800°C, ordinary stainless steel begins to fail. Carbides precipitate at grain boundaries. Corrosion resistance drops. Strength fades. But for components inside jet engines, furnace systems, and nuclear reactors, failure is not an option.
This is where SS347 and SS348 stainless steels earn their place. These austenitic grades use niobium and tantalum stabilization to resist the carbide precipitation that plagues standard stainless steels like SS304 in high-heat environments. They retain 70% of room-temperature strength at 650°C —a critical margin for components that must perform reliably under thermal stress.
Yet machining these alloys presents real challenges. Their high alloy content increases work hardening tendencies. Cutting forces run 10–15% higher than SS304. Tool wear accelerates. Chip control becomes difficult.
This guide walks you through the practical realities of CNC machining SS347 and SS348. You will learn the material properties that matter, the cutting parameters that work, and the strategies that separate successful machining from costly scrap.
What Makes SS347 and SS348 Different?
Chemical Composition and Stabilization
Both SS347 and SS348 belong to the austenitic stainless steel family. But their stabilization chemistry sets them apart.
SS347 contains 17–19% chromium, 9–13% nickel, and 0.8–1.0% niobium. The niobium content is specified as a minimum of 8 times the carbon content. This ensures that carbon bonds with niobium instead of chromium, preventing the formation of chromium carbides that cause intergranular corrosion after welding or high-temperature exposure.
SS348 shares the same base composition but adds 0.1–0.3% tantalum and imposes stricter limits on cobalt, boron, and phosphorus. These tighter controls make SS348 the preferred choice for nuclear applications where trace elements could become radioactive under neutron bombardment.
Mechanical and Physical Properties
| Property | SS347 | SS348 |
|---|---|---|
| Tensile strength | 515 MPa | 515 MPa |
| Yield strength | 205 MPa | 205 MPa |
| Hardness (annealed) | 18–22 HRC | 18–22 HRC |
| High-temp strength (650°C) | 70% of RT | 72% of RT |
| Magnetic response | Non-magnetic | Non-magnetic |
Both grades maintain useful strength at temperatures up to 800°C . SS348 offers slightly better creep resistance due to its stricter impurity limits. Neither grade becomes brittle after welding—a critical advantage over unstabilized grades.
Why Stabilization Matters
Without stabilization, welded stainless steel develops a phenomenon called intergranular corrosion. During welding, chromium carbides form at grain boundaries in the heat-affected zone (HAZ) . The chromium is pulled from solid solution, leaving chromium-depleted zones that corrode rapidly.
SS347 and SS348 avoid this. Niobium and tantalum form carbides preferentially, leaving chromium in solution where it belongs. The result: weldments that retain corrosion resistance without post-weld annealing.
How Do You Machine SS347 and SS348?
Core Machining Operations
CNC milling of SS347 requires attention to direction. Climb milling—where the cutter rotates in the direction of feed—is preferred over conventional milling. This reduces the time the cutter spends rubbing against work-hardened surfaces, lowering cutting forces by 10–15% .
CNC turning works well for cylindrical components like exhaust stacks and tubing fasteners. Moderate feed rates balance material removal against heat buildup. Heavy cuts can generate enough heat to work-harden the surface, making subsequent passes difficult.
CNC drilling and boring present the greatest challenges. The high ductility of these alloys causes chips to pack in flutes, leading to tool breakage. Peck drilling—withdrawing the drill periodically to clear chips—is essential for deep holes.
Optimal Cutting Parameters
| Operation | Cutting Speed (m/min) | Feed Rate (mm/rev) | Depth of Cut (mm) |
|---|---|---|---|
| Milling (carbide) | 100–180 | 0.10–0.20 | 1–3 |
| Turning (carbide) | 120–200 | 0.12–0.25 | 1.5–4 |
| Drilling (carbide) | 80–140 | 0.08–0.15 | 1–2.5 |
These parameters reflect a key reality: SS347 machines 10–15% slower than SS304 . Attempting SS304 speeds leads to rapid tool wear, work hardening, and scrap.
What Tools Work Best for These Alloys?
Tool Material Selection
Carbide tools are the baseline for machining SS347 and SS348. High-speed steel dulls too quickly against these alloys.
Within carbide grades, fine-grain carbide with 6–8% cobalt offers the best combination of toughness and wear resistance. The fine grain structure maintains sharp edges longer than standard carbide grades.
Coatings Make the Difference
Uncoated carbide wears rapidly against work-hardening stainless steels. AlTiN (Aluminum Titanium Nitride) coatings provide the best protection, extending tool life by 40–60% compared to uncoated tools.
AlTiN performs well because:
- It remains stable at temperatures up to 800°C
- Its high hardness resists abrasive wear
- Its low friction reduces heat generation
Tool Geometry Matters
Positive rake angles of 5–10° reduce cutting forces. Sharp edges minimize the deformation that leads to work hardening. For roughing, round inserts with large radii distribute cutting forces. For finishing, square inserts with honed edges produce clean surfaces.
Rigid tool holding is non-negotiable. Shrink-fit holders minimize deflection under the high cutting forces these alloys generate. Any tool movement translates to dimensional errors or chatter.
Coolant Delivery
Standard coolant delivery does not suffice. High-pressure coolant at 70–120 bar directed precisely at the cutting zone:
- Reduces heat buildup that softens tools
- Flushes chips before they can be re-cut
- Prevents work hardening by keeping temperatures down
A shop machining thick-walled boiler components switched to high-pressure coolant and saw tool life increase by 30% while cycle times dropped due to reduced manual chip clearing.
How Do You Control Chips and Surface Finish?
Chip Control Strategies
SS347 and SS348 produce stringy, tough chips that wrap around tools and cause damage. Poor chip control leads to:
- Re-cutting that accelerates work hardening
- Heat buildup from chips packed against the tool
- Tool breakage when chips bind
Effective strategies include:
- Chip breaker tools designed specifically for austenitic stainless steels
- Feed rates high enough to produce short, curly chips rather than long ribbons
- High-speed chip conveyors that remove chips immediately
Surface Finish Requirements
Surface finish matters for high-temperature components. Rough surfaces trap contaminants and provide initiation points for oxidation and cracking.
| Application | Target Ra (μm) |
|---|---|
| Jet engine components | ≤0.8 |
| Furnace parts | ≤1.6 |
| Boiler casings | ≤3.2 |
Achieving these finishes requires:
- Sharp tools—dull tools burnish rather than cut, raising surface roughness
- Consistent feeds—variation shows as surface marks
- Proper coolant—lubrication reduces built-up edge that scars surfaces
Quality Control Methods
Coordinate Measuring Machines (CMM) verify dimensional accuracy. For components that must fit with other parts in high-temperature assemblies, even small deviations matter.
Profilometers measure surface roughness. A 0.8 μm Ra finish feels smooth to the touch, but the profilometer confirms it quantitatively.
High-temperature tensile testing ensures that machined parts retain the strength specified for operating conditions. This matters most for critical components like jet engine parts and pressure vessel fittings.
What Post-Machining Processes Are Required?
Heat Treatment
SS347 and SS348 require minimal heat treatment compared to other alloys, but two processes deserve attention.
Solution annealing at 1040–1150°C followed by water quenching dissolves carbides and ensures a uniform austenitic structure. This is typically performed before machining, as it softens the material to 18–22 HRC and enhances corrosion resistance.
Stress relief annealing at 300–500°C for 1–2 hours reduces residual stresses from machining. For large components like boiler casings, this prevents distortion during high-temperature service.
Cleaning and Surface Treatments
Ultrasonic cleaning removes coolant residues and chips from complex geometries. Any contamination left on a component can cause pitting or scaling when the part reaches operating temperature.
Passivation restores the chromium oxide layer on the surface. While optional, it provides an extra margin of corrosion resistance for parts exposed to both high heat and moisture.
Shot peening adds surface compressive stress that improves fatigue life by 20–30% . For components subject to thermal cycling, this can mean years of additional service life.
Where Are These Materials Used?
Aerospace Applications
Jet engines operate at extreme temperatures. Collector rings, exhaust stacks, and engine mounting hardware machined from SS347 withstand thermal cycling that would crack less stable alloys.
One aerospace manufacturer reports that SS347 components outperform SS304 by 30% in creep resistance at 700°C —a margin that translates directly to engine life and maintenance intervals.
Industrial Furnaces and Boilers
Boiler casings, furnace heating elements, and annealing box covers see continuous exposure to temperatures up to 800°C. SS347 resists scaling and deformation that would require frequent replacement in lesser materials.
Nuclear Industry
SS348's stricter impurity limits make it the specified material for nuclear reactor components. Cobalt and boron controls prevent the formation of radioactive isotopes under neutron bombardment. Tantalum stabilization provides additional creep resistance for long-term service.
Chemical Processing
Tubing fasteners, metal bellows, and expansion joints in high-temperature chemical systems require both corrosion resistance and mechanical integrity. SS347 delivers both, maintaining strength in environments that would weaken standard stainless steels.
What Standards Govern These Materials?
| Standard | Scope |
|---|---|
| ASTM A240 | Sheet and plate |
| ASTM A276 | Bars and shapes |
| ASTM A269 | Welded and seamless tubing |
| ASME SA-240M | Pressure vessel components |
| ISO 15510 | Chemical composition |
| EN 1.4550 | European equivalent to SS347 |
Compliance with these standards ensures that the material delivered matches the specifications that engineers rely on. For critical applications, material test reports (MTRs) documenting chemistry and mechanical properties are essential.
What Challenges Will You Face?
Work Hardening
SS347 work-hardens more aggressively than SS304. Each pass hardens the surface, making subsequent passes more difficult. The solution: use sharp tools, maintain consistent feed rates, and avoid light cuts that rub instead of cut.
A shop machining jet engine components reduced work hardening issues by switching to AlTiN-coated tools and increasing feed rates . The sharper edges and deeper cuts minimized the time the tool spent deforming the surface.
Heat Generation
Friction generates heat. Heat softens tools and accelerates work hardening. The solution: high-pressure coolant directed precisely at the cutting zone.
One manufacturer reported 30–40% longer tool life after implementing 100-bar coolant delivery. The investment in high-pressure systems paid back in reduced tooling costs and fewer unplanned tool changes.
Tool Wear
Abrasion and heat combine to wear tools rapidly. The solution: AlTiN-coated carbide tools that withstand both.
While coated tools cost 2–3 times more than uncoated carbide, their 40–60% longer life reduces total tooling expense in production runs. For small batches, the productivity gains may justify the higher price regardless.
Yigu Technology's Perspective
At Yigu Technology, we machine SS347 and SS348 regularly for clients in aerospace, industrial equipment, and energy markets. Our experience confirms that success depends on three factors: tool selection, parameter control, and coolant strategy.
We use AlTiN-coated carbide tools with high-pressure coolant systems delivering 100 bar to the cutting zone. Our data shows this combination reduces tool wear by 50% compared to standard setups—a difference that matters for both cost and quality.
For high-temperature parts, we recommend solution annealing before machining. This ensures uniform machinability and eliminates the variations that cause problems in subsequent operations.
We comply with ASTM A240 and ASME SA-240M, with 100% CMM inspection to verify dimensional accuracy. When you need components that will perform at 800°C, we deliver the consistency that makes that performance possible.
Conclusion
SS347 and SS348 stainless steels fill a critical role in high-temperature applications. Their niobium and tantalum stabilization prevents the intergranular corrosion that limits other grades. Their high-temperature strength keeps components reliable when temperatures climb.
But machining these alloys demands respect. Their work hardening tendencies require sharp tools, consistent feeds, and aggressive coolant strategies. Their higher cutting forces demand rigid setups and stable machines.
The trade-off is worth it. Components machined from SS347 and SS348 deliver 2–3 times the service life of SS304 in high-temperature environments. For applications where failure is not an option, that margin justifies the additional machining care.
Choose the right tools. Control the parameters. Manage the heat. Your components will perform when it matters most.
FAQ
What distinguishes SS348 from SS347?
SS348 contains 0.1–0.3% tantalum in addition to niobium stabilization, plus stricter limits on cobalt, boron, and phosphorus. These tighter controls make SS348 suitable for nuclear applications where impurity levels must be minimized to prevent radiation activation.
Why are SS347 and SS348 preferred for high-temperature applications?
Their niobium and tantalum stabilization prevents carbide precipitation at grain boundaries during welding or high-temperature exposure. This maintains both corrosion resistance and mechanical strength up to 800°C —far beyond the capabilities of unstabilized grades like SS304.
How does machining SS347 compare to SS304?
SS347 is harder to machine than SS304 due to higher work hardening tendencies and higher alloy content. Expect to reduce cutting speeds by 10–15% and use more durable AlTiN-coated carbide tools. The higher machining cost is justified by superior high-temperature performance in the final application.
What cutting parameters work best for SS347?
For turning, use 120–200 m/min cutting speed with 0.12–0.25 mm/rev feed. For milling, 100–180 m/min with 0.10–0.20 mm per tooth. Use climb milling to minimize work hardening. Always use high-pressure coolant directed at the cutting zone.
Can SS347 and SS348 be welded without post-weld heat treatment?
Yes. The niobium and tantalum stabilization in these grades prevents carbide precipitation in the heat-affected zone. This means post-weld annealing is typically not required to restore corrosion resistance—a significant advantage over unstabilized grades.
Contact Yigu Technology for Custom Manufacturing
Need precision-machined components in SS347, SS348, or other high-temperature alloys? Yigu Technology delivers quality you can trust for demanding applications. Our capabilities include multi-axis CNC machining, high-pressure coolant systems, and comprehensive quality control with CMM verification.
We serve aerospace, industrial equipment, energy, and custom manufacturing clients who require consistent performance at elevated temperatures. Let our experience with stabilized stainless steels work for your next project.
Contact Yigu Technology today to discuss your requirements or request a quote.
