Material Selection for Edge Welded Bellows
Material choice is the single most critical decision in bellows design. The right material determines cycle life, vacuum compatibility, corrosion resistance, and operating temperature range. At Alpha Technology, we work primarily with three materials — each optimized for specific application requirements. For dimensional options in each material, see our Welded Bellows Specs.
Material Comparison at a Glance
| Property | SS316L | AM350 | Inconel 718 |
|---|---|---|---|
| Material Type | Austenitic Stainless Steel | Precipitation-Hardening Stainless |
Nickel-Chromium Superalloy |
| Max Operating Temp | ~450°C | ~500°C | ✅ ~700°C |
| Cryogenic Performance | ✅ Excellent (to -269°C) |
Good | ✅ Excellent |
| Corrosion Resistance | ✅ Excellent |
Moderate | ✅ Excellent |
| Magnetic Properties | Slightly Magnetic (cold-worked) |
Magnetic | ✅ Non-Magnetic |
| Fatigue Resistance | Good | ✅ Best in Class |
Very Good |
| Relative Cost | $$ | $$$ | $$$$ |
| Best For | General UHV, Semiconductor, Chemical |
High Cycle, Aerospace, Dynamic Seals |
High Temp, Oxidizing, Cryogenic |
SS316L — The Industry Workhorse
SS316L is the most widely used bellows material and the default choice for semiconductor and general vacuum applications. Its low carbon content (<0.03%) prevents carbide precipitation during welding, preserving corrosion resistance at the weld seam.
Key Advantages
- Best corrosion resistance of the three — handles
aggressive process gases (Cl₂, HCl, WF₆) - Excellent weldability with micro-plasma processes
- Readily available in thin foils (0.05mm–0.3mm) for diaphragm stamping
- Proven track record in UHV chambers at 10⁻¹⁰ Torr
- Most cost-effective for standard vacuum applications
Limitations
- Fatigue life limited compared to AM350 under high-stress cycling
- Becomes slightly magnetic after cold working — avoid if zero magnetism
required - Not suitable above ~450°C due to creep
Typical Applications
Semiconductor wafer handling, gate valve seals, vacuum manipulators, CVD/PVD chamber bellows, general industrial vacuum.
AM350 — Maximum Fatigue Life
AM350 is a precipitation-hardening stainless steel that offers the highest fatigue resistance among bellows materials. Through controlled heat treatment, AM350 achieves exceptional spring properties and strength-to-weight ratio — making it the material of choice for high-cycle dynamic applications.
Key Advantages
- Superior cycle life — 2-5x longer than SS316L under
identical stroke conditions - Higher strength allows thinner walls = more flexible convolutions
- Excellent spring rate stability over millions of cycles
- Good performance up to ~500°C
Limitations
- Moderate corrosion resistance — not ideal for aggressive chemical
environments - Magnetic — unsuitable for MRI or sensitive instrumentation
- More expensive than SS316L
- Requires precise heat treatment after welding for optimal properties
Typical Applications
Aerospace actuators, satellite propulsion bellows, high-cycle vacuum manipulators, cryogenic dynamic seals, missile guidance systems.
Inconel 718 — Extreme Environment Specialist
Inconel 718 is a nickel-chromium superalloy engineered for the harshest conditions. When your application involves temperatures above 500°C, oxidizing atmospheres, or cryogenic extremes, Inconel is the answer. It retains mechanical strength at temperatures where stainless steels soften.
Key Advantages
- Maximum temperature capability — stable mechanical
properties to ~700°C - Outstanding oxidation and creep resistance at elevated temperatures
- Non-magnetic — ideal for MRI, NMR, and particle physics
applications - Excellent corrosion resistance in harsh environments including
seawater - Performs from cryogenic (-269°C) to high temperature without brittle
transition
Limitations
- Most expensive — 3-5x the cost of SS316L
- More difficult to weld — requires specialized procedures
- Longer lead times for thin-foil material sourcing
Typical Applications
Nuclear reactor seals, particle accelerator beamline bellows, aerospace engine components, high-temperature chemical processing, cryogenic vacuum systems.
How to Choose
| Start with SS316L if… |
Your application needs general vacuum performance with good corrosion resistance at a reasonable cost. This covers 70%+ of all bellows applications. |
| Choose AM350 if… |
Cycle life is your bottleneck. You need millions of cycles with stable spring rates, and your environment is not chemically aggressive. |
| Choose Inconel 718 if… |
Temperature, oxidation, or non-magnetic requirements push beyond what stainless steel can handle. |
Not sure which
material fits your application?