What Is 1.4021 (ASTM 420 / SS2303) Stainless Steel?
Grade 1.4021, also written as AISI 420, ASTM A276 Type 420 and SS 2303, is a martensitic hardenable stainless steel supplied as round bar, rod and machined shafting. Its chromium content of 12-14 % builds a passive surface film, while a carbon range of roughly 0.16-0.25 % allows the steel to be hardened and tempered to high strength. The grade is magnetic in every condition, which makes it easy to separate from non-magnetic austenitic bars on site.
Because hardness is developed by heat treatment rather than by cold work, 1.4021 bar is normally delivered in the quenched and tempered condition, widely designated QT 700. This condition balances strength, ductility and toughness, and it avoids the residual-stress problems that appear when a martensitic grade is used in the as-rolled state.
Chemical Composition of 1.4021 / EN X20Cr13
The table below lists the typical ladle analysis of grade 1.4021 (X20Cr13) according to EN 10088-3. Small differences exist between the EN, ASTM and former SS 2303 specifications, and the applicable analysis is always stated on the mill certificate.
| Element | Typical range (%) | Function |
|---|---|---|
| Carbon (C) | 0.16-0.25 | Controls hardness after quenching |
| Silicon (Si) | max. 1.00 | Deoxidation |
| Manganese (Mn) | max. 1.50 | Hardenability, sulphur control |
| Phosphorus (P) | max. 0.040 | Impurity limit |
| Sulphur (S) | max. 0.030 | Impurity limit |
| Chromium (Cr) | 12.0-14.0 | Corrosion resistance, hardening |
Chromium is the decisive element: below roughly 12 % the passive film becomes unstable in moist air, while higher carbon improves hardness at the cost of some corrosion resistance and weldability. That trade-off explains why 1.4021 sits between the softer 1.4006 (410) and the more highly alloyed 1.4034 (AISI 440C class) grades.
Mechanical Properties and the QT 700 Condition
| Property | Typical value in the QT 700 condition |
|---|---|
| 0.2 % proof strength, Rp0.2 | 700 MPa minimum |
| Tensile strength, Rm | 800-1000 MPa |
| Elongation, A | 12 % minimum |
| Hardness | approx. 240-290 HBW |
| Impact toughness | Moderate, and it falls as hardness rises |
| Magnetic behaviour | Ferromagnetic in all conditions |
The figures above are indicative values for the quenched and tempered delivery condition; the exact properties depend on bar diameter and on the tempering temperature selected by the producer. Because 1.4021 hardens through a martensitic transformation, large cross-sections cool more slowly and may develop lower core hardness than small ones.
Heat Treatment, Welding and Corrosion Behaviour
Hot forming is carried out in the range 1100-800 °C followed by slow cooling, and the bar is then heat treated to the required strength. A typical hardening cycle for 1.4021 uses an austenitising temperature of about 950-1000 °C with an oil or forced-air quench, after which tempering in the range 600-750 °C produces the QT 700 condition. Tempering below roughly 400 °C should be avoided because it leaves the steel in a highly stressed and brittle state.
For machining, the quenched and tempered structure of 1.4021 is generally easier to cut than the work-hardening austenitic grades, which is one reason the grade remains popular for high-volume turned parts such as shafts, spindles and piston rods.
1.4021 can be welded with consumables of matching composition, typically after preheating to about 200-300 °C, and the joint is normally tempered after welding to restore ductility and relieve stress. Thick sections benefit from a post-weld heat treatment because martensitic stainless steels are sensitive to cold cracking. Corrosion resistance is good in air, steam, fresh water, certain alkaline solutions and mildly aggressive chemicals. The grade is nevertheless not intended for marine service or for chloride-bearing environments, where pitting and stress corrosion cracking can develop.
Typical Applications
Pump and valve components
Shafts, spindles and piston rods
Stirrers and agitator parts
Bolts, nuts and fasteners
Fittings, valve stems and trim
General machined parts needing strength with moderate corrosion resistance
Standard availability is machined round bar in the range 16-160 mm, and other profiles or cut lengths can be produced from the same heat.
Frequently Asked Questions
Q: Is 1.4021 stainless steel magnetic?
Yes. As a martensitic grade it is ferromagnetic in every condition, unlike 304 or 316 austenitic stainless steel.
Q: Are 1.4021, AISI 420 and SS 2303 the same material?
They describe the same family of 12-14 % chromium martensitic stainless steel under European, American and Nordic designations. Small tolerance differences exist, so the governing specification is stated on the mill certificate.
Q: Can 1.4021 round bar be welded?
It can, using matching consumables and preheat of roughly 200-300 °C, followed by tempering to restore toughness. Heavy sections always need post-weld heat treatment.
Q: Can 1.4021 be used in seawater?
No. The grade resists fresh water and mildly aggressive media but is not suited to marine or chloride-rich service, where pitting and stress corrosion cracking occur.
Q: What is the difference between 1.4021 and 304?
1.4021 is hardenable, magnetic and considerably stronger after heat treatment, while 304 is austenitic, non-magnetic and offers better corrosion resistance but lower strength.
Q: In which condition is 1.4021 bar supplied?
It is normally supplied in the quenched and tempered QT 700 condition, ready for machining and for further heat treatment when higher hardness is required.
