What ASTM A519 Grade 4520 Tubing Is
ASTM A519 grade 4520 is a carburizing nickel-chromium-molybdenum alloy steel produced as seamless mechanical tubing and registered under the UNS number G45200. It sits in the same family as the other nickel-chromium-molybdenum carburizing grades, but carries a higher chromium range of 0.70–0.90%, which raises hardenability and gives a firmer, more uniform case response on thicker sections.
The steel is designed around a low-carbon, tough core combined with a hard, wear-resistant surface produced by carburizing. That structure is what gear teeth, splined sleeves and other contact-loaded components need: bending fatigue is carried by the core, while pitting and wear resistance come from the carburized case.
Chemical Composition
| Element | ASTM A519 grade 4520 |
| Carbon | 0.18–0.23% |
| Manganese | 0.45–0.65% |
| Silicon | 0.15–0.35% |
| Nickel | 1.65–2.00% |
| Chromium | 0.70–0.90% |
| Molybdenum | 0.20–0.30% |
| Phosphorus | 0.035% max |
| Sulphur | 0.040% max |
Compared with a grade such as 4320, the additional chromium makes 4520 slower to lose hardness as the section thickens, so carburized components in larger sizes can still be quenched to a sound case and core. Nickel supports core toughness and case ductility, while molybdenum improves hardenability and limits temper embrittlement.
Heat Treatment and Case Properties
Carburized parts are specified by case depth, surface hardness and core hardness rather than by a single tensile value. Typical practice is to carburize in the region of 900–930 °C, quench and then temper at low temperature to relieve stress without softening the case. Surface hardness is commonly agreed in the 58–62 HRC range, and case depth is selected according to contact load and wall thickness.
Because the tube is thin walled compared with a solid gear blank, quenching distortion has to be managed. Vacuum or low-pressure carburizing with high-pressure gas quenching gives a more uniform cooling rate than an oil quench and reduces ovality and length change, which in turn cuts the amount of stock that must be left for grinding. Where oil quenching is used, the component is designed and machined with distortion allowance and finished after heat treatment.
For the tubing as supplied, ASTM A519 tabulates minimum tensile strength, yield strength and elongation for each grade and delivery condition, so the certificate values depend on whether the tube is hot finished, cold drawn or heat treated.
Typical Applications
Planetary, ring and pinion gears in industrial and wind turbine gearboxes
Traction and reduction gear components in heavy transport equipment
Splined sleeves, coupling hubs and drive collars with hardened flanks
Contact and cam components in rotating and reciprocating machinery
Heavily loaded bushes and journals requiring a hard surface on a tough core
Machining and Inspection Points
Machining is carried out in the annealed condition, before carburizing, using sharp tooling and adequate feed to control built-up edge on the soft low-carbon structure. Stock is left on all surfaces that must stay accurate after quenching. Welding is not part of the normal process route for these components, since the alloy content makes the heat affected zone hard and prone to cracking.
Inspection of the finished component covers case depth by sectioning and hardness traverse, surface hardness, core hardness, microstructure and dimensional checks. Where surface integrity matters, magnetic particle examination is used to detect grinding cracks, and gear flank geometry is verified against the drawing.
FAQ
Q: What is UNS G45200?
It is the unified numbering system identifier for the 4520 nickel-chromium-molybdenum chemistry that ASTM A519 lists among its standard grades. The UNS number identifies the composition, while ASTM A519 sets the tubing requirements.
Q: How does 4520 differ from 4320 tubing?
Both are carburizing nickel-chromium-molybdenum grades. Grade 4520 contains more chromium, roughly 0.70–0.90% against 0.40–0.60% in 4320, which improves hardenability and gives more reliable case and core properties on heavier sections.
Q: What case depth is normally specified for 4520 components?
Case depth is chosen from the contact load and the wall or tooth thickness, then written on the drawing as an effective case depth with a tolerance. It is verified after heat treatment by sectioning a sample or a sacrificial component.
Q: Can 4520 tubing be welded?
It is not a welded construction grade. Components are normally machined and carburized, and any joining is carried out by mechanical means. The alloy content makes the heat affected zone hard and crack sensitive if welded after hardening.
Q: How is distortion controlled during carburizing?
By leaving machining stock on accurate surfaces, by using a uniform quench such as high-pressure gas quenching where distortion must be minimal, and by finishing the component after heat treatment by grinding or hard turning.
Q: How should grade 4520 tubing be ordered?
State the grade and UNS number, the delivery condition of the tube, the machined component dimensions, the required case depth and hardness, the tolerance to be held after heat treatment and the examination requirements.
