QT400-18 is a widely specified grade of ductile cast iron defined under the Chinese national standard GB/T 1348. The designation outlines its core performance metrics: QT indicates spherical graphite nodular iron, 400 denotes a minimum tensile strength of 400 MPa, and 18 represents a minimum elongation percentage of 18%. Belonging to the ferritic class of nodular irons, this material combines high impact resistance, fracture toughness, and structural ductility.
The microstructure consists of spherical graphite nodules embedded within a predominantly ferritic matrix. The rounded graphite geometry eliminates the internal stress concentrations typical of standard gray iron flakes, allowing the alloy to achieve mechanical behavior similar to cast carbon steel while retaining the superior castability of iron. As a result, QT400-18 is selected for load-bearing components operating under heavy shock loads, vibration, or sub-zero temperatures.
This technical guide provides an overview of QT400-18 ductile cast iron. The following sections outline the international material standards, chemical composition limits, mechanical properties, structural advantages, and common industrial applications.
Table of Contents
International Standards and Material Equivalents
Global engineering standards classify ductile cast iron grades based on mechanical properties such as minimum tensile strength, yield strength, and elongation percentage. QT400-18 under GB/T 1348 corresponds to several international material specifications designed for high-ductility, high-impact applications.
| Standard Organization | Standard Code | Grade Designation |
| International | ISO 1083 | JS/400-18 / 400-18 |
| European Union | EN 1563 | EN-GJS-400-18 / EN-JS1020 |
| United States | ASTM A536 | Grade 60-40-18 |
| Japan | JIS G5502 | FCD400-18 |
| Germany (Legacy) | DIN 1693 | GGG-40.3 / GGG-40 |
While these standards share similar mechanical thresholds, subtle variations exist regarding low-temperature impact testing requirements and allowable chemical impurity limits. For instance, EN-GJS-400-18U or ASTM A536 Grade 60-40-18 require strict control of trace elements to guarantee minimum Charpy V-notch impact toughness values at -20°C or -40°C. Cross-referencing material callouts requires verification of both room-temperature tensile requirements and specified impact test temperatures.
Chemical Composition of QT400-18 Ductile Cast Iron
The mechanical properties of QT400-18 depend directly on strict control of the molten metal chemistry. Chemical elements are balanced to promote a fully ferritic matrix while preventing the formation of embrittling phases or irregular graphite shapes.
| Element | Weight Percentage (%) |
| Carbon (C) | 3.50 – 3.90 |
| Silicon (Si) | 2.20 – 2.80 |
| Manganese (Mn) | ≤ 0.30 |
| Phosphorus (P) | ≤ 0.04 |
| Sulfur (S) | ≤ 0.02 |
| Magnesium (Mg) | 0.03 – 0.06 |
| Rare Earth Elements (RE) | 0.01 – 0.03 |
Maintaining a carbon content between 3.50% and 3.90% provides appropriate fluidity during casting and generates sufficient graphite volume. Silicon acts as a strong graphitizing agent, promoting carbon precipitation as graphite rather than iron carbide during solidification.
Manganese, phosphorus, and sulfur levels are restricted to low limits. Manganese stabilizes pearlite, so keeping its concentration below 0.30% supports the formation of a soft, ductile ferritic matrix. High phosphorus creates brittle iron phosphide networks at grain boundaries, which lowers impact toughness. Low sulfur prevents excessive consumption of magnesium during the nodulizing treatment.
Magnesium and rare earth elements serve as nodulizing agents added prior to pouring. These additions transform the flake graphite growth habit into smooth, spherical Type I and Type II nodules, granting high strength and elongation.
Mechanical Properties of QT400-18
The mechanical performance of QT400-18 stems from its combination of tensile strength, yield behavior, and high elongation. Room temperature testing guidelines specified in GB/T 1348 define standard mechanical parameters:
| Property | Standard Requirement |
| Tensile Strength ( | ≥ 400 MPa |
| Yield Strength ( | ≥ 250 MPa |
| Elongation ( | ≥ 18% |
| Hardness | 130 – 180 HBW |
| Charpy V-Notch Impact Energy (RT) | ≥ 18 J |
| Charpy V-Notch Impact Energy (-20°C / -40°C) | ≥ 12 J (QT400-18AL grade) |
A minimum elongation value of 18% separates QT400-18 from higher-strength ductile iron grades such as QT500-7 or QT600-3. The fully ferritic microstructure provides energy absorption under mechanical shock loads. Specialized variants designated as QT400-18AL or QT400-18L maintain Charpy impact toughness down to -20°C or -40°C through controlled silicon levels and minimized trace impurities.
Key Benefits of QT400-18 Ductile Iron Castings
QT400-18 offers a combination of mechanical toughness, physical stability, and production efficiency that separates it from standard gray cast irons and forged structural steels. The fully ferritic matrix and high nodule count provide operational advantages in demanding structural and mechanical applications.
Exceptional Ductility and Fracture Toughness
A minimum elongation of 18% allows QT400-18 to exhibit stress-strain behavior similar to low-carbon cast steels. Spherical graphite nodules act as arrestors against micro-crack propagation under heavy cyclic stress. High energy absorption prevents sudden catastrophic failure when components experience unexpected impact, overload, or dynamic shock loads in service.
Superior Machineability and Reduced Tool Wear
The soft ferritic metallic structure surrounding free graphite nodules lowers shear resistance during cutting operations. Free graphite provides natural lubrication at the tool-chip interface, reducing friction and thermal buildup. High-speed turning, milling, and tapping operations achieve extended cutting tool life and superior surface finish parameters compared to pearlitic iron grades or forged steel components.
Effective Vibration Damping and Damping Capacity
Graphite nodules disperse stress waves throughout the iron matrix, offering higher vibration damping capacity than structural steel. Dynamic machinery components cast from QT400-18 absorb operational chatter, reduce structure-borne noise, and lower high-frequency resonance, protecting mating gear assemblies and bearings from premature wear.
Sub-Zero Low-Temperature Impact Performance
Strict control of phosphorus, manganese, and silicon limits preserves Charpy impact values in cold environments. Low-temperature variants such as QT400-18AL maintain ductile behavior down to -20°C or -40°C without undergoing a ductile-to-brittle transition, making the alloy suitable for arctic infrastructure and outdoor heavy equipment.
Superior Near-Net Shape Castability and Cost Savings
A low melting point (approximately 1150°C to 1200°C) and high molten metal fluidity allow complex, thin-walled geometries to be poured with fewer casting defects. Near-net shape casting capability reduces overall raw material consumption, minimizes secondary machining time, and lowers total unit production costs compared to multi-piece steel fabrications or drop-forged components.
Casting Processes for QT400-18 Ductile Iron
Production of QT400-18 components requires selecting a molding method based on part geometry, required dimensional tolerances, surface finish specifications, and production volume. The inherent fluidity of QT400-18 supports several standard iron casting techniques.
Sand Casting
Sand casting represents the most common method used for producing medium to large QT400-18 components. Green sand molding handles high-volume production for automotive brackets, axle housings, and machinery frames. Resin-bonded sand (no-bake sand) provides higher mold rigidity, making it suitable for heavy structural parts, pump casings, and large valve bodies where dimensional stability is needed to prevent wall movement during solidification.
Shell Mold Casting
Shell mold casting uses resin-coated sand applied to a heated metal pattern to form a thin, rigid mold shell. This process balances production speed with tighter dimensional tolerances than conventional green sand casting. Shell molding is applied to medium-volume runs of transmission components, hydraulic manifolds, and engine brackets.
Investment Casting
Investment casting produces small to medium-sized QT400-18 components requiring high dimensional precision, thin wall sections, and fine surface finishes. Wax patterns are coated with ceramic slurry to create a seamless shell mold. Investment casting reduces or eliminates subsequent machining steps for intricate valve internals, complex linkages, and specialized hardware.
Lost Foam Casting
Lost foam casting uses an expanded polystyrene pattern coated with a refractory wash, which is placed inside unbonded sand supported by vibration. Molten QT400-18 iron vaporizes the foam pattern upon contact, taking its exact shape within the sand mold cavity. This process eliminates draft angles and core prints, allowing complex geometries, internal passages, and multi-part integrated structures to be poured in a single operation. Lost foam casting is frequently applied to engine blocks, gearbox housings, and pump bodies requiring dimensional precision without extensive machining.
Typical Industrial Applications
The high ductility, shock absorption, and castability of QT400-18 make it a widely chosen material across multiple heavy industries. Components cast from QT400-18 operate under cyclic loads, pressure containment requirements, and harsh environmental conditions.
Automotive and Commercial Vehicles
QT400-18 provides the fracture resistance needed for safety-critical chassis and drivetrain parts. Typical applications include steering knuckles, control arms, brake calipers, wheel hubs, differential housings, and engine mounting brackets subjected to severe road vibration and shock loads.
Agricultural Machinery
Agricultural equipment relies on QT400-18 for components exposed to heavy mechanical impact and soil abrasion. Cast parts include tractor axle housings, hitch assemblies, gearbox frames, cultivator brackets, plowing hubs, and combine harvester support arms.
Railway and Transit Systems
High impact toughness and fatigue resistance support railway infrastructure and rolling stock safety. Railway castings include rail clip baseplates, axle boxes, brake disc hubs, bogie suspension brackets, and shock absorber mountings operating under severe wheel-rail dynamic forces.
Mining and Construction Equipment
Heavy mining and construction equipment operates under extreme shock loads, fluctuating stress cycles, and harsh environmental conditions requiring high resistance to brittle fracture. QT400-18 provides structural reliability and low-temperature impact toughness for heavy-duty earthmoving machinery. Key applications include excavator link arms, loader linkage arms, crusher bearing housings, track rollers, conveyor sheaves, hydraulic excavator swivel joints, crane counterweight brackets, concrete pump elbows, and grader pivot housings.
Oil and Gas
Upstream and midstream installations utilize QT400-18 for pressure-retaining and structural hardware requiring impact resistance in cold outdoor climates. Common oil and gas casting parts include mud pump fluid ends, pipe hangers, sucker rod pump jacks, cable sheaves, and wellhead support brackets.
Pump and Valve
High pressure-tightness and fluid corrosion resistance make QT400-18 standard for fluid-handling machinery. Applications include high-pressure pump casings, impellers, butterfly valve discs, check valve bodies, gate valve housings, and municipal water pipeline fittings.
Conclusion
QT400-18 ductile cast iron combines a fully ferritic matrix with high nodule counts to offer an optimal balance of high ductility, fracture toughness, and superior machineability. Matching the performance of low-carbon cast steels while retaining the fluid castability and vibration damping of iron, this material addresses structural and pressure-retaining requirements across demanding industrial sectors.
Production of high-quality custom iron castings requires strict chemical control, proper nodulizing treatment, and modern molding technology. As an established metal casting manufacturer in China, SIMIS manufactures custom iron casting components to exact international specifications. Comprehensive casting capability across sand casting, investment casting, lost foam casting, and shell mold casting supports net-shape production for global OEMs across automotive, agricultural, railway, pump, and valve industries.
Frequently Asked Questions (FAQ)
This section addresses common metallurgical and manufacturing queries regarding QT400-18 ductile cast iron, covering material comparisons, processing requirements, and chemical composition parameters.
What is the main difference between QT400-18 and QT450-10 ductile iron?
The primary difference lies in their microstructure and resulting mechanical balance. QT400-18 features a fully ferritic matrix, providing higher elongation (≥18%) and superior impact toughness, especially in cold environments. QT450-10 contains a higher proportion of pearlite, which increases tensile strength to 450 MPa but reduces elongation to 10%. QT400-18 is selected when shock absorption and fracture resistance are prioritized, whereas QT450-10 is chosen for higher yield strength and wear resistance.
Is QT400-18 ductile iron weldable?
Weldability is limited due to the risk of forming brittle martensite or free carbides in the heat-affected zone. Preheating the component to 200°C–350°C and using specialized nickel-based welding rods allows repair or joining operations. Post-weld annealing treatment is recommended to restore the fully ferritic matrix and relieve residual cooling stresses.
Why is low phosphorus content specified for QT400-18?
Phosphorus forms a low-melting ternary eutectic known as steadite along grain boundaries during solidification. Elevated phosphorus levels create brittle steadite networks that severely lower impact toughness and room-temperature elongation. Keeping phosphorus concentrations below 0.04% maintains high energy absorption and prevents intergranular fracture under shock loading.
What heat treatment is applied to achieve 18% elongation in QT400-18?
When as-cast cooling conditions fail to produce a fully ferritic matrix, a full ferritizing annealing process is performed. Castings are heated to 890°C–920°C to break down pearlite and iron carbides, held at temperature, and then furnace-cooled slowly to 600°C before air cooling. This converts residual pearlite into ferrite and graphite, maximizing structural ductility to meet or exceed the 18% elongation threshold.
