QT400 Steel: Properties and Key Applications

QT400 steel, classified as a quenched and tempered (Q&T) steel, is a medium-carbon alloy steel known for its high yield strength of approximately 400 MPa. This steel grade is primarily used in applications requiring high strength and toughness, making it suitable for structural components in various industries. The primary alloying elements in QT400 steel include carbon (C), manganese (Mn), silicon (Si), and chromium (Cr), each contributing to the steel's mechanical properties and overall performance.

Comprehensive Overview

QT400 steel is categorized as a medium-carbon alloy steel, which typically contains carbon content ranging from 0.2% to 0.6%. The alloying elements play a crucial role in enhancing the steel's properties. For instance, manganese improves hardenability and tensile strength, while silicon enhances deoxidation and increases strength at elevated temperatures. Chromium contributes to corrosion resistance and hardenability.

The most significant characteristics of QT400 steel include its excellent mechanical properties, such as high tensile strength, good toughness, and wear resistance. These properties make it ideal for applications that require high strength-to-weight ratios and durability under stress.

Advantages and Limitations

Advantages:
- High Strength: QT400 exhibits a yield strength of around 400 MPa, making it suitable for heavy-duty applications.
- Good Toughness: The tempering process enhances toughness, allowing it to withstand impact loads.
- Versatile Applications: Its properties make it suitable for various engineering applications, including construction and manufacturing.

Limitations:
- Weldability Challenges: Due to its high strength, QT400 can be challenging to weld without proper preheating and post-weld treatment.
- Cost Considerations: The alloying elements can increase production costs compared to lower-grade steels.

QT400 steel holds a significant position in the market due to its balance of strength and toughness, making it a popular choice in industries such as construction, automotive, and machinery manufacturing.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	QT400	USA	Closest equivalent to EN S355J2
ASTM	A572 Grade 50	USA	Similar mechanical properties but different chemical composition
EN	S355J2	Europe	Minor compositional differences to be aware of
DIN	1.0570	Germany	Equivalent with slight variations in yield strength
JIS	SM490A	Japan	Comparable but with different impact requirements
ISO	6300	International	General classification for structural steels

When selecting QT400 steel, it is essential to consider these equivalents as they may have subtle differences in composition and mechanical properties that could affect performance in specific applications.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.20 - 0.25
Mn (Manganese)	1.20 - 1.60
Si (Silicon)	0.15 - 0.40
Cr (Chromium)	0.30 - 0.50
P (Phosphorus)	≤ 0.025
S (Sulfur)	≤ 0.015

The primary role of key alloying elements in QT400 steel includes:
- Carbon (C): Increases hardness and strength through solid solution strengthening and precipitation hardening.
- Manganese (Mn): Enhances hardenability and tensile strength, improving the steel's performance under stress.
- Chromium (Cr): Improves corrosion resistance and contributes to the steel's hardenability, making it suitable for various environments.

Mechanical Properties

Property	Condition/Temper	Test Temperature	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Quenched & Tempered	Room Temp	400 - 600 MPa	58 - 87 ksi	ASTM E8
Yield Strength (0.2% offset)	Quenched & Tempered	Room Temp	400 MPa	58 ksi	ASTM E8
Elongation	Quenched & Tempered	Room Temp	20%	20%	ASTM E8
Reduction of Area	Quenched & Tempered	Room Temp	50%	50%	ASTM E8
Hardness (Brinell)	Quenched & Tempered	Room Temp	300 - 350 HB	30 - 35 HRC	ASTM E10
Impact Strength (Charpy)	Quenched & Tempered	-20°C (-4°F)	30 J	22 ft-lbf	ASTM E23

The combination of these mechanical properties makes QT400 steel particularly suitable for applications that require high strength and toughness, such as structural components in buildings, bridges, and heavy machinery. Its ability to withstand significant loads while maintaining structural integrity is a key consideration in engineering design.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	Room Temp	7.85 g/cm³	0.284 lb/in³
Melting Point	-	1425 - 1540 °C	2600 - 2800 °F
Thermal Conductivity	Room Temp	50 W/m·K	34.5 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temp	0.46 kJ/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	Room Temp	0.0000017 Ω·m	0.0000017 Ω·in
Coefficient of Thermal Expansion	Room Temp	12 x 10⁻⁶ /K	6.67 x 10⁻⁶ /°F

The practical significance of key physical properties includes:
- Density: The relatively high density of QT400 steel contributes to its strength and durability, making it suitable for heavy structural applications.
- Thermal Conductivity: Its moderate thermal conductivity allows for effective heat dissipation in applications involving high temperatures.
- Coefficient of Thermal Expansion: The low coefficient of thermal expansion ensures dimensional stability under varying temperature conditions, which is critical in structural applications.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3%	25°C (77°F)	Fair	Risk of pitting corrosion
Sulfuric Acid	10%	25°C (77°F)	Poor	Not recommended
Sea Water	-	25°C (77°F)	Fair	Moderate resistance
Atmospheric	-	-	Good	Requires protective coating

QT400 steel exhibits moderate corrosion resistance, particularly in atmospheric conditions. However, it is susceptible to pitting corrosion in chloride environments and should not be used in applications involving strong acids like sulfuric acid. Compared to other steel grades, such as stainless steels, QT400's corrosion resistance is limited, making it less suitable for marine or highly corrosive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400 °C	752 °F	Suitable for high-temperature applications
Max Intermittent Service Temp	500 °C	932 °F	Short-term exposure only
Scaling Temperature	600 °C	1112 °F	Risk of oxidation beyond this limit
Creep Strength considerations	400 °C	752 °F	Begins to degrade at elevated temperatures

At elevated temperatures, QT400 steel maintains its strength but may experience oxidation and scaling, which can affect its performance in high-temperature applications. Proper surface treatments or coatings can enhance its resistance to oxidation.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG Welding	ER70S-6	Argon + CO₂	Preheat recommended
TIG Welding	ER70S-2	Argon	Post-weld heat treatment needed
Stick Welding	E7018	-	Requires careful control to avoid cracking

QT400 steel can be welded using various processes, but it requires careful consideration of preheating and post-weld heat treatment to avoid cracking. The use of appropriate filler metals is crucial to ensure compatibility and maintain mechanical properties.

Machinability

Machining Parameter	QT400 Steel	AISI 1212	Notes/Tips
Relative Machinability Index	60	100	QT400 is less machinable than AISI 1212
Typical Cutting Speed (Turning)	30 m/min	60 m/min	Use carbide tools for best results

QT400 steel has moderate machinability, which can be improved with proper tooling and cutting conditions. Carbide tools are recommended for effective machining.

Formability

QT400 steel exhibits limited formability due to its high strength. Cold forming is possible but may require significant force, while hot forming is more feasible. The steel's work hardening characteristics should be considered during forming operations to avoid cracking.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Quenching	850 - 900 °C / 1562 - 1652 °F	30 min	Water or Oil	Hardening
Tempering	400 - 600 °C / 752 - 1112 °F	1 - 2 hours	Air	Toughness improvement

During heat treatment, QT400 steel undergoes significant metallurgical transformations. Quenching increases hardness, while tempering reduces brittleness and enhances toughness, resulting in a balanced combination of strength and ductility.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Construction	Structural beams	High strength, good toughness	Essential for load-bearing structures
Automotive	Chassis components	High yield strength, impact resistance	Durability under dynamic loads
Machinery	Heavy equipment frames	Wear resistance, structural integrity	Long service life under stress

Other applications include:
- Mining equipment
- Agricultural machinery
- Offshore structures

QT400 steel is chosen for these applications due to its high strength-to-weight ratio, making it ideal for components that must endure heavy loads while minimizing weight.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	QT400 Steel	AISI 4140	S355J2	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High yield strength	Moderate yield strength	Moderate yield strength	QT400 offers superior strength
Key Corrosion Aspect	Fair resistance	Good resistance	Moderate resistance	QT400 is less corrosion-resistant
Weldability	Moderate	Good	Good	QT400 requires careful welding practices
Machinability	Moderate	Good	Moderate	QT400 is less machinable than AISI 4140
Formability	Limited	Good	Moderate	QT400 is less formable due to high strength
Approx. Relative Cost	Moderate	Moderate	Low	Cost may vary based on market conditions
Typical Availability	Moderate	High	High	QT400 may be less readily available

When selecting QT400 steel, considerations include its mechanical properties, cost-effectiveness, and availability. While it offers excellent strength and toughness, its challenges in weldability and machinability must be addressed during design and fabrication. Additionally, QT400's performance in corrosive environments may necessitate protective coatings or alternative materials in specific applications.