AISI 347 is a stabilized austenitic stainless steel designed for applications where corrosion resistance, high-temperature strength, and resistance to intergranular corrosion are important. It is commonly stabilized with niobium, also known as columbium, which helps prevent chromium carbide precipitation during exposure to elevated temperatures. This makes AISI 347 different from standard 304 stainless steel and gives it useful performance in welded structures, heat exchangers, exhaust components, and other industrial parts that operate under demanding thermal conditions.

AISI 347 is closely related to AISI 304 but provides better resistance to sensitization after exposure to temperatures in the approximate range of 800°F to 1500°F. During heating and cooling, chromium carbides can form at grain boundaries in conventional austenitic stainless steels. This can reduce the local chromium content and make the material more vulnerable to intergranular corrosion. The niobium stabilization in AISI 347 binds carbon and reduces this risk. For this reason, AISI 347 is often selected for components that may experience repeated heating, welding, or high-temperature service.

The material has an austenitic microstructure and maintains good toughness over a wide temperature range. It also offers good general corrosion resistance in many industrial environments. Its chromium content provides a protective passive oxide layer, while nickel helps maintain the stable austenitic structure. AISI 347 can resist oxidation and corrosion better than many carbon and low-alloy steels, although the exact performance depends strongly on the chemical environment, temperature, surface condition, and component design.

AISI 347 is commonly used in aerospace components, chemical processing equipment, petroleum processing systems, heat exchangers, high-temperature piping, exhaust systems, furnace components, and welded assemblies. It can also be used for CNC machined components where both dimensional accuracy and thermal stability are important. Typical machined parts include bushings, flanges, shafts, fittings, adapters, valve components, mounting parts, brackets, and custom precision components.

CNC machining AISI 347 requires careful control of cutting conditions because stainless steel can work harden during machining. If a cutting tool repeatedly rubs against the material instead of producing a stable chip, the machined surface can become harder. This hardened layer makes subsequent cutting more difficult and may increase tool wear. Using sharp carbide tools, suitable cutting speeds, controlled feed rates, and sufficient coolant can help maintain stable machining conditions.

Tool engagement is particularly important when machining AISI 347. A cutting tool should remove material efficiently rather than remain in contact with the surface without cutting. Excessive tool rubbing can increase heat generation and contribute to work hardening. Rigid machine setups, secure workholding, and appropriate tool geometry are useful for reducing vibration and improving dimensional consistency. For thin-wall or small precision parts, machining strategy becomes especially important because excessive cutting force can cause deformation.

Drilling AISI 347 also requires attention to heat and chip evacuation. A dull drill or insufficient feed can cause rubbing and heat buildup. Through-coolant or an appropriate external coolant system can improve chip removal when producing deeper holes. For precision holes, machining parameters should be selected according to the hole diameter, depth, tool material, and required tolerance. Threading operations may also require rigid tool control because stainless steel can generate considerable cutting resistance.

Turning and milling AISI 347 can produce good surface quality when the machining process is stable. Carbide inserts with suitable chip breakers are commonly used for production work. For milling complex components, the toolpath should avoid unnecessary dwell and repeated passes over the same area. Finishing passes should use controlled engagement to achieve the required dimensional accuracy without generating excessive heat.

Surface treatment for AISI 347 depends on the final application. Unlike aluminum, AISI 347 does not normally require anodizing to provide corrosion resistance. Its natural chromium-rich passive film already provides a degree of protection. However, additional surface finishing can improve appearance, cleanliness, wear resistance, or corrosion performance depending on the service environment.

Mechanical polishing is often selected when a smoother and cleaner surface is required. Grinding and polishing can reduce surface roughness and remove machining marks. For components used in chemical, food, pharmaceutical, or other cleanliness-sensitive applications, a properly finished surface can make cleaning easier and reduce locations where contaminants may accumulate. The required surface roughness should be specified according to the actual application rather than selecting an unnecessarily fine finish.

Passivation is another important surface treatment for AISI 347 stainless steel. CNC machining, grinding, handling, and fabrication can leave free iron or other contaminants on the surface. A suitable passivation process can remove surface contamination and support the formation of a more uniform passive layer. Passivation is especially useful when corrosion resistance and surface cleanliness are important. The process should be selected according to the part geometry, required cleanliness, and applicable industry requirements.

Electropolishing can also be considered when a very smooth and clean stainless steel surface is required. It removes a controlled amount of material from the surface and can improve surface smoothness while reducing microscopic roughness. This treatment is useful for certain precision components where cleanliness and corrosion resistance are important. However, dimensional changes caused by material removal should be considered when tight tolerances are involved.

AISI 347 can also be supplied with different levels of surface finish before CNC machining. When purchasing material for precision components, engineers should consider the material condition, dimensions, heat treatment history, and required mechanical properties. For critical components, material certification may also be required to verify chemical composition and mechanical properties.

The combination of CNC machining and surface treatment allows AISI 347 to be used for demanding custom components. Engineers should define the material grade, part dimensions, critical tolerances, surface roughness, heat exposure, and operating environment before production. Features such as deep holes, thin walls, internal threads, sharp corners, and narrow grooves can affect machining difficulty and should be reviewed during the design stage.

For precision AISI 347 parts, design for manufacturability is also important. Avoiding unnecessarily deep cavities, providing suitable tool access, and using realistic tolerances can reduce machining time and improve consistency. Tight tolerances should be applied only to functional features. If a component will later be polished, passivated, or electropolished, the effect of the surface treatment on dimensions should also be considered during CNC machining.

AISI 347 offers a useful balance of corrosion resistance, weldability, toughness, and high-temperature performance. Its niobium stabilization makes it particularly suitable for applications involving elevated temperatures and welding where resistance to intergranular corrosion is important. CNC machining requires attention to work hardening, heat generation, tool condition, and chip control, but properly planned processes can produce accurate and repeatable components. Surface treatments such as polishing, passivation, and electropolishing can further improve the functional performance and appearance of finished parts. For engineers and manufacturers requiring custom stainless steel components, AISI 347 remains a practical choice for applications where both precision manufacturing and reliable performance in demanding environments are required.