Comparing Two Strong Steels: Alloy Steel vs Stainless Steel for CNC machining Eleanore)

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Alloy steel and stainless steel are two very strong and versatile metals commonly used in CNC machining. But which one is ultimately better suited for different applications? In this article, we’ll compare the properties, strengths, uses, and machining characteristics of alloy steel versus stainless steel.
What is Alloy Steel?
Alloy steel is composed of iron, carbon, and other alloying elements like chromium, silicon, nickel, molybdenum, and manganese. Varying the amounts of these alloying elements produces alloy steels with different properties optimized for specific applications. Alloy steels are stronger, harder, and more wear resistant than plain carbon steel. Common types of alloy steel include 4140, 4340, H13, D2, A2, tool steel, and more.
Alloy steel is valued in machining for its high strength, hardness, impact toughness, and fatigue resistance. It holds up well under high levels of stress and resists abrasion. Alloy steel is commonly used for parts like gears, shafts, tools, pumps, and molds.
What is Stainless Steel?
Stainless steel contains at minimum 10.5% chromium along with iron and carbon. The chromium forms an invisible passive layer of chromium oxide on the surface that protects the steel from corrosion and rusting. There are many types of stainless steel including ferritic, austenitic, martensitic, duplex, and precipitation hardening. 304 and 316 are the most common grades used.
Stainless steel is valued for its excellent corrosion and oxidation resistance. It maintains its strength and durability in harsh environments and can be sterilized for use in medical, food, and pharmaceutical applications. However, stainless steel has lower hardness and is not as strong overall compared to alloy steel.
Key Differences in Properties
- Strength: Alloy steel has higher strength than stainless steel. Alloy steels like 4140 have a tensile strength around 900-1000 MPa compared to 500-700 MPa for 304 stainless steel.
- Hardness: Alloy steel also has greater hardness, measuring 20-50 HRC for alloy steel versus 25-35 HRC for stainless steel. The increased carbon content in alloy steel lends more hardness.
- Toughness: Stainless steel is tougher and more ductile than high-carbon alloy steel. The nickel content boosts toughness in stainless, making it less brittle.
- Corrosion Resistance: Stainless steel is much more resistant to corrosion, rust, and oxidation thanks to its chromium content. Alloy steel can rust easily.
- Wear Resistance: Alloy steel has better wear resistance and abrasion resistance compared to stainless steel due to its hardness.
- High Temperature Use: Stainless steel maintains its strength and resists scaling and warping better than alloy steel at elevated temperatures above 1000°F.
Machining Characteristics
The differences in properties between alloy steel and stainless steel translate to differences in how they machine.
- Cutting Speeds/Feeds: Alloy steels can be machined faster than stainless steels. Their lower hardness and toughness allows higher cutting speeds and feed rates. Stainless steels have a gummier cut requiring conservative speeds and feeds.
- Tool Wear: The harder alloy steels cause more rapid tool wear compared to softer stainless steels. Carbide tooling holds up better against alloy steels.
- Surface Finish: Stainless steel generally produces a nicer cosmetic surface finish compared to alloy steel due to gummier cutting.
- Chip Control: Stainless steel creates long stringy chips that must be controlled. Alloy steel chips are more rigid and break up easier.
- Built-Up Edge: Stainless steel is prone to form a tenacious built-up edge on cutting tools that requires frequent cleaning/tool changes to mitigate.
- Coolants: Coolants/lubricants are more important when machining stainless steel to get good chip control and surface finishes.
Ideal Uses for Each Metal
- Alloy Steel: Gears, shafts, pumps, tooling, molds, agricultural parts, engine components
- Stainless Steel: Medical devices, food handling and processing equipment, marine components, chemical and pharmaceutical industry, architecture
How to Remove Chrome Plating
Chrome plating provides a shiny, protective chromium coating on metallic surfaces. It is common on automobile trim, household fixtures, and other items. But chrome eventually wears off and needs to be removed and reapplied. Here are some tips for removing chrome plating:
Caustic Bath:
- Make a bath containing a caustic soda like sodium hydroxide or a strong alkali like lye. Concentrations of 25-50% caustic soda work well.
- Immerse the chrome plated part in the heated caustic bath. Use appropriate PPE like gloves and eye protection.
- Allow the part to soak until the plating blisters and separates from the substrate, usually a few minutes up to one hour.
- Remove and rinse the part thoroughly with water.
- The substrate now needs polished and re-plated.
Abrasive Blasting:
- Media blast the chrome plated surface with an abrasive like aluminum oxide or garnet.
- Start with lower psi (60-80 psi) to avoid digging into the base metal.
- Increase blasting pressure if needed.
- Rotate and position the part to remove all chrome plating.
- Avoid over-blasting as it will damage the underlying surface.
Mechanical Stripping:
- Use a grinder, sander, or buffing wheel to mechanically strip and grind off the chrome plating.
- Take care when getting close to substrate to avoid removal of base material.
- Finish with a finer abrasive pad.
- Part will need polished afterwards to smooth surface.
Acid Bath:
- Make a dilute sulfuric or hydrochloric acid bath, 10-20% concentration.

- Immerse and soak chrome plated parts to dissolve the chrome coating.
- Use proper ventilation and follow safety precautions when handling acids.
- Dispose of spent acid bath properly after use.
Removing chrome plating takes time but is doable on a small scale with the right chemicals, equipment, and safety measures. Proper disposal of waste is critical as well. CNC Milling CNC Machining