1.4021 QT800 vs. 1.4021 QT700: Strength Comparison

Understanding 1.4021 and the +QT Designation

1.4021 is the EN 10088 designation for X20Cr13, a martensitic stainless steel with 12-14% chromium and 0.16-0.25% carbon. It is equivalent to AISI 420, UNS S42000, DIN X20Cr13, ISO X20Cr13 and the Chinese grade 20Cr13. The suffixes +QT700 and +QT800 describe the delivery condition: the steel is quenched and tempered to a specified strength class. Importantly, the number after +QT designates the minimum tensile strength Rm in megapascals, not the yield strength: +QT700 guarantees Rm of 700 MPa minimum, and +QT800 guarantees Rm of 800 MPa minimum. This follows the EN 10088-3 / ISO 683-13 convention for heat-treated martensitic grades.

Chemical Composition and Equivalents

The composition below follows EN 10088-3 for 1.4021 bar; the same limits apply to the equivalent grades in other systems.

Element	1.4021 (EN 10088-3)
Carbon (C)	0.16-0.25%
Silicon (Si)	≤ 1.00%
Manganese (Mn)	≤ 1.00%
Phosphorus (P)	≤ 0.040%
Sulfur (S)	≤ 0.030%
Chromium (Cr)	12.00-14.00%

Standard System	Equivalent Grade
EN	1.4021 (X20Cr13)
AISI / UNS	420 / S42000
DIN	X20Cr13
ISO	X20Cr13+QT700 / +QT800
GB (China)	20Cr13

Mechanical Properties: QT700 vs QT800

The guaranteed mechanical values below follow EN 10088-3 for the two strength classes. The same steel is tempered at different temperatures to reach each class.

Property	1.4021 +QT700	1.4021 +QT800
Tensile strength Rm	700-850 MPa	800-950 MPa
0.2% proof stress Rp0.2	≥ 450 MPa	≥ 600 MPa
Elongation A	≥ 15%	≥ 12%
Impact toughness	Higher (softer temper)	Lower (harder temper)
Wear resistance	Moderate	Improved

Note that hardness is not separately guaranteed by EN 10088-3 for these classes; it is the result of the selected temper. In practice QT700 material measures roughly 200-230 HBW and QT800 roughly 230-260 HBW, and these figures should be confirmed on the mill certificate.

Heat Treatment and Metallurgy

Both classes start with austenitising at 950-1000 °C followed by oil or polymer quenching. The difference is the tempering temperature: a higher temper around 650-700 °C softens the martensite to reach the QT700 class, while a lower temper around 600-650 °C retains more hardness and reaches QT800. When tempering 13% chromium martensitic steel, avoid the range roughly between 450 °C and 600 °C where toughness can drop due to temper embrittlement. For maximum corrosion resistance, the steel should be hardened, tempered and then finished by grinding or polishing, because the passive film forms best on a clean surface.

Applications and Selection Guidance

1.4021 +QT700 suits shafts, bolts, pump components and machine parts where ductility and toughness matter and loads are moderate. 1.4021 +QT800 suits valve stems, turbine blades, spindles and highly stressed parts where higher strength and wear resistance justify lower ductility. If the part is welded, note that martensitic 13% chromium steel requires preheating and post-weld tempering; for weld-heavy designs a lower-carbon grade or an austenitic alternative should be considered. If the environment is corrosive, 420-type steel is limited to mild conditions and should be coated or avoided in favour of an austenitic grade.

FAQ

What does +QT800 mean on 1.4021?

It means the bar is delivered in the quenched and tempered condition with a guaranteed minimum tensile strength Rm of 800 MPa. The number after +QT is the tensile strength class, not the yield strength.

Is 1.4021 the same as AISI 420?

Yes. 1.4021 (X20Cr13) is the EN designation of AISI 420; the UNS number is S42000 and the Chinese equivalent is 20Cr13.

Which is better: QT700 or QT800?

There is no absolute better grade. QT700 offers better ductility and toughness; QT800 offers higher strength, hardness and wear resistance. The choice depends on whether the part fails by overload or by wear.

Can QT classes be welded?

Only with precautions. Preheat to 200-300 °C, use low-hydrogen filler and temper after welding. Even so, the heat-affected zone will be hard and crack-sensitive; for welded assemblies consider a lower-carbon martensitic grade or an austenitic stainless steel.

What is the difference between 1.4021 and 1.4028?

1.4028 (X30Cr13) has 0.26-0.35% carbon, about 0.10% higher than 1.4021. It reaches higher hardness after hardening but has lower toughness and weldability. 1.4021 is the more general-purpose of the two.

Is hardness guaranteed for QT classes?

EN 10088-3 guarantees tensile properties for the QT classes; hardness is a result of the temper and is reported on the mill certificate. If a hardness window is critical, specify it as an additional requirement on the purchase order.