AISI 8622 is a nickel-chromium-molybdenum carburizing steel supplied as seamless mechanical tubing under ASTM A519. With carbon controlled at 0.20 to 0.25 percent, it delivers a harder core after case hardening than 8620 while retaining the case properties required for gears and shafts. This article covers the grade chemistry, the conditions and hardness ranges listed in A519, the heat treatment routes used in practice, and the applications where 8622 tubing is specified.
What ASTM A519 Covers
ASTM A519 covers hot-finished or cold-finished seamless carbon and alloy steel mechanical tubing. Unlike pressure piping standards, A519 tubing is specified primarily for machined mechanical components rather than for fluid service. The standard covers manufacturing, tolerances, surface condition and, where ordered, mechanical requirements. Tensile properties are not mandatory for every grade; when they are required, they are stated on the purchase order and verified by testing.
8622 Alloy Design and Chemistry
8622 belongs to the 8600 family of carburizing steels, in which nickel, chromium and molybdenum are balanced for through-hardening response and case-core compatibility. The chemistry is registered as UNS G86220 and falls within the following limits:
| Element | 8622, percent |
| Carbon | 0.20 to 0.25 |
| Manganese | 0.70 to 0.90 |
| Phosphorus, max | 0.025 |
| Sulfur, max | 0.040 |
| Silicon | 0.15 to 0.35 |
| Nickel | 0.40 to 0.70 |
| Chromium | 0.40 to 0.60 |
| Molybdenum | 0.15 to 0.25 |
The slightly higher carbon content relative to 8620 raises the core hardness and strength after carburizing, which is why 8622 is selected for gears and shafts that carry heavy bending loads through the tooth root or the spline.
Conditions and Hardness in ASTM A519
A519 defines several supply conditions for alloy mechanical tubing, including hot finished, cold drawn, annealed, normalized and normalized and tempered. For each grade the standard lists hardness ranges for the common conditions. For 8622, the annealed condition is typically delivered in the 180 to 230 HB band, while cold-drawn or turned-and-polished material runs higher depending on the reduction. The exact range for the ordered condition is stated on the mill test certificate.
Heat Treatment Routes for 8622 Tubing
8622 is a carburizing grade, so the final component properties come from a case-hardening cycle applied after machining:
Pretreatment - normalize at approximately 920 to 950 degrees C and temper at 620 to 660 degrees C to condition the core microstructure and improve machinability.
Carburizing - gas carburizing at 880 to 930 degrees C, or low-pressure vacuum carburizing where distortion control matters; effective case depths of 0.5 to 2.0 mm are typical for gear applications.
Quenching - oil quench from 820 to 850 degrees C after carburizing, or a reheat quench to refine the grain before final tempering.
Tempering - a low-temperature temper at 160 to 200 degrees C relieves quenching stress while preserving case hardness.
After a correctly controlled cycle, the carburized case is typically in the HRC 58 to 64 range and the core retains sufficient toughness for impact loading. Where retained austenite must be minimized for dimensional stability, a sub-zero treatment between quenching and tempering is applied.
8622 vs 8620
| Property | 8620 | 8622 |
| Carbon, percent | 0.18 to 0.23 | 0.20 to 0.25 |
| Nickel, percent | 0.40 to 0.70 | 0.40 to 0.70 |
| Chromium, percent | 0.40 to 0.60 | 0.40 to 0.60 |
| Molybdenum, percent | 0.15 to 0.25 | 0.15 to 0.25 |
| Core hardness after carburizing | Baseline | Higher, due to the extra carbon |
| Machinability | Better | Slightly lower |
Both grades are hardened with the same family of processes. 8622 is chosen when the design needs the additional core strength; 8620 remains the default where machinability and cost dominate.
Typical Applications
Because 8622 tubing is supplied in hollow form, it is often machined directly into tubular components: transmission gears and pinions, shafts with integral splines, couplings, spindles, bearing races and hydraulic components. The seamless hollow section eliminates the cost and waste of drilling a solid bar, and the carburized case provides wear resistance on the working surfaces while the tougher core carries the load.
Sourcing Considerations for 8622 Mechanical Tubing
Specify the exact condition (annealed, cold drawn, normalized) on the purchase order, because A519 hardness ranges differ by condition.
Order to outside diameter and wall thickness; A519 tolerances are tighter than pipe standards, which matters for near-net machining.
Request the mill test certificate with full chemistry and, if specified, the tensile and hardness results.
Confirm straightness and surface finish where the tube feeds directly into an automatic machining line.
Frequently Asked Questions
Is 8622 a standard grade under ASTM A519?
Yes. 8622 is listed in ASTM A519 as an alloy steel mechanical tubing grade, registered as UNS G86220, with the chemistry shown in the table above.
What is the difference between 8622 and 8620 steel?
The carbon range is higher in 8622 (0.20 to 0.25 versus 0.18 to 0.23 percent). Nickel, chromium and molybdenum are the same, so 8622 develops a harder core after carburizing while 8620 machines more easily.
Can 8622 tubing be welded?
It is a hardenable alloy steel and can be welded with preheat and low-hydrogen practice, but 8622 is normally used for machined mechanical parts rather than welded piping systems. Welding is followed by tempering or a full heat treatment when the joint must carry load.
What hardness can be expected after carburizing 8622?
With a correctly controlled carburizing, quench and temper cycle, the case hardness is typically HRC 58 to 64, depending on case depth and carbon potential.
How is A519 tubing different from A106 or A53 pipe?
A106 and A53 are pressure piping standards with hydrostatic testing and pressure-focused requirements. A519 is a mechanical tubing standard for machined components, specified by outside diameter and wall thickness with tolerance and surface requirements aimed at machining rather than pressure service.
What size range is available in 8622 seamless tubing?
Seamless mechanical tubing in A519 is commonly available from about 6 mm to 200 mm outside diameter, with wall thicknesses chosen to suit the finished component; larger sizes can be supplied hot finished on request.
