What 4718 Is
4718 is the AISI designation of a nickel-chromium-molybdenum alloy steel, supplied as seamless mechanical tubing to ASTM A519, the standard for seamless carbon and alloy steel mechanical tubing. The grade is used for components that need a strong, tough core and, where specified, a hardened case, such as heavy-duty gears, pinions, shafts, and transmission parts. It belongs to the same family as 8620 and 4320 but with a different balance of nickel, chromium, and molybdenum, giving its own combination of hardenability, toughness, and cost. The material number in the UNS system is G47180.
Chemical Composition
The composition of AISI 4718 per ASTM A519 is:
| Element | Range or maximum, % |
|---|---|
| Carbon | 0.16 – 0.21 |
| Manganese | 0.70 – 0.90 |
| Phosphorus, max | 0.035 |
| Sulphur, max | 0.040 |
| Silicon | 0.15 – 0.35 |
| Nickel | 0.90 – 1.20 |
| Chromium | 0.35 – 0.55 |
| Molybdenum | 0.30 – 0.40 |
The carbon content of about 0.18% suits both case hardening and through hardening of small sections. The nickel provides toughness, the chromium adds hardenability, and the molybdenum raises the strength and the resistance to tempering. Composition figures that add vanadium or that change the chromium and molybdenum far from these values belong to modified or different grades and should not be used for AISI 4718.
Heat Treatment and Properties
4718 can be supplied annealed or normalized for machining, or hardened and tempered for service. For through-hardened parts, the steel is quenched from about 830 to 860 C and tempered at 540 to 650 C to set the strength-toughness balance; typical properties for medium sections are a tensile strength of 800 to 950 MPa and a yield strength of 650 to 750 MPa, with the exact values depending on the section and the temper. For wear resistance, the steel is case hardened by carburizing, which gives a hard, wear-resistant surface over a tough core. The delivered properties are agreed between the buyer and the mill and verified by tensile testing; claims of strengths far above the normal range of the grade should be verified against the certified test results.
Applications
4718 mechanical tubing is machined into gears, pinions, shafts, couplings, and transmission components in the automotive, construction, agricultural, and general machinery industries. The tube form is used for hollow parts and where machining from tube is more economical than from bar. For heavy-duty gearing that must combine case hardness with core toughness, 4718 is a cost-effective alternative to the higher-alloy carburizing steels.
Machining and Welding
In the annealed condition, 4718 machines well with standard tooling. After hardening it is ground or hard-turned. Welding requires preheat of about 200 to 250 C and post-weld heat treatment for critical joints, and welded assemblies cannot match the properties of the hardened base metal. For welded fabrications, a lower-carbon grade is usually preferred. The steel is commonly carburized or carbonitrided for gear teeth and bearing surfaces, with the case depth specified by the design.
Ordering Notes
When ordering A519 4718 tubing, state the standard, the grade, the outside diameter and wall thickness, the length, the delivery condition, and the required mechanical properties and testing. The certificate reports the heat analysis and the agreed test results. For hardened parts, confirm the hardenability and the heat-treatment route with the heat treater, and agree the non-destructive testing for critical components.
Frequently Asked Questions
Q: What is AISI 4718?
A nickel-chromium-molybdenum alloy steel supplied as mechanical tubing to ASTM A519, used for gears, shafts, and transmission components.
Q: What is the composition of 4718?
0.16 to 0.21% carbon, 0.90 to 1.20% nickel, 0.35 to 0.55% chromium, and 0.30 to 0.40% molybdenum, with manganese 0.70 to 0.90%.
Q: How is 4718 heat treated?
Annealed or normalized for machining, hardened and tempered for strength, or carburized for a wear-resistant case over a tough core.
Q: What is 4718 used for?
Heavy-duty gears, pinions, shafts, couplings, and transmission components in machinery.
Q: Can 4718 be welded?
With preheat and post-weld heat treatment, but welded joints do not match the hardened base-metal properties. Lower-carbon grades are preferred for welding.
Q: Is 4718 the same as 4340?
No. 4340 has much higher carbon, nickel, and molybdenum, giving deeper hardenability and higher potential strength. 4718 is a lighter Ni-Cr-Mo grade with better machinability and case-hardening response.
