How to increase the elongation of QT450 ductile iron to over 22%?

How can we increase the elongation to over 22% while maintaining the same tensile strength? This requires starting from the "microstructure" and making refined process adjustments. 

Core idea: Maximize the plasticity and toughness of the matrix while maintaining sufficient strength. Specifically, it means obtaining as much ferrite matrix as possible while ensuring the high quality of graphite balls. The following are specific technical routes and measures: First, precise adjustment of chemical composition (basic). The current QT450 composition may only be for the purpose of "meeting standards", and to achieve high elongation, it is necessary to develop towards "high purification" and "balance". 

1. Carbon Equivalent: Moderately increase, lean towards high carbon strategy: While ensuring no graphite floating, try to increase the carbon content (recommended 3.6% -3.9%) and control the silicon content appropriately. This can increase the number of graphite balls, improve thermal conductivity, reduce solidification shrinkage, and is beneficial for improving strength and plasticity. The carbon equivalent (CE) is recommended to be controlled between 4.3% and 4.5%. 

2. Silicon: Control the final silicon content strategy: Silicon is a solid solution strengthening element, and excessive silicon will significantly reduce plasticity. On the premise of ensuring ferrite formation, control the final silicon content (silicon content after pouring) at a lower level of 2.2% -2.5%. To achieve this, low silicon spheroidizing agents can be used and silicon can be added through inoculants. 

3. Manganese: Extreme Reduction (Key!) Strategy: Manganese is a stable element in pearlite and is highly prone to segregation at grain boundaries, forming brittle phases and being the "number one killer" of elongation. The manganese content must be reduced from the conventional<0.3% to<0.15%, with an ideal state of<0.10%. This is the most effective and economical chemical method to achieve an elongation rate of over 22%. 

4. Phosphorus and sulfur: Ultimate purification of phosphorus: Formation of brittle phosphorus eutectic. Goal: ≤ 0.03%, the lower the better. Sulfur: Consuming spheroidizing agents and generating inclusions. The sulfur content of the original molten iron before spheroidization is ≤ 0.012%. 

5. Interference elements: Strictly control and monitor elements such as titanium, chromium, vanadium, tin, antimony, etc. They can stabilize pearlite or form harmful carbides. 

The use of spheroidizing agents containing trace amounts of rare earths (cerium, lanthanum) can neutralize their harmful effects.

 2、 Strengthening the spheroidization and incubation process (core) is a decisive step in improving the quality and quantity of graphite balls. 

1. Spheroidization treatment: Pursuing stability and softness. Spheroidizing agent: Selecting low magnesium, low rare earth, and high-purity spheroidizing agents. For example, a spheroidizing agent with a Mg content of 5% -6% can reduce the tendency of white casting and shrinkage stress caused by excessive magnesium. Process: Using methods such as capping and wire feeding to ensure smooth spheroidization reaction, stable absorption rate, and reduced magnesium light dust. 

2. Fertility treatment: The key objective is to significantly increase the number of graphite balls to over 150/mm ² and improve the roundness of the balls. Fertility agent: Use efficient fertility agents, such as those containing strontium, barium, and zirconium, which have strong anti-aging ability and good nucleation effect. Craftsmanship: "Multiple incubation" must be used! One pregnancy: carried out inside the spheroidization bag. Secondary/Accompanying Pregnancy: This is of utmost importance! During pouring, the fine particle inoculant is uniformly added with the iron water flow through a dedicated feeder. It can provide a large number of instantaneous crystalline cores, which is the core means to increase the number of graphite spheres. Intratype incubation: If conditions permit, set incubation blocks in the pouring system for the third incubation. 

3、 Optimize the melting and cooling process 

1 Smelting: Using high-purity pig iron and clean scrap steel to control harmful elements from the source. It is recommended to set the tapping temperature between 1530-1560 ℃ and allow it to stand at a suitable high temperature to facilitate the upward movement of inclusions. 

2. Cooling rate: For thin-walled parts, accelerating cooling may be beneficial for increasing pearlite and improving strength, but it is not conducive to elongation. For QT450 that pursues high elongation, the cooling rate should be appropriately reduced, such as using insulation risers, thickening sprues, optimizing casting processes (such as using resin sand instead of metal molds), etc., to promote the formation of ferrite and the full growth of graphite. 

4、 Heat treatment: The most reliable guarantee is that if the as cast properties are still unstable after the above process adjustments (especially due to uneven wall thickness causing pearlite in some areas), then ferritization annealing is the most reliable method to achieve an elongation rate of over 22%. 

Process route: 

1 High temperature stage: Heat to 900-920 ℃ and hold for 1-3 hours (depending on wall thickness). The purpose is to transform all pearlite into austenite. 

2. Medium temperature stage: Slowly cool (or directly move) the furnace to 700-730 ℃ and keep it warm for 2-4 hours. This stage is crucial as it allows sufficient time for supersaturated carbon in austenite to precipitate onto the original graphite spheres, thereby fully transforming into ferrite. 

3. Discharge from the furnace: Afterwards, it can be cooled to below 600 ℃ and discharged from the furnace for air cooling. Effect: After this treatment, the matrix structure can reach over 95% ferrite, with an elongation rate easily exceeding 22%. At the same time, due to the presence of graphite balls and solid solution strengthening of silicon, the tensile strength can still remain stable at over 450MPa. 

Summary and Action Roadmap 

1. Diagnosis Status: Firstly, analyze the metallographic structure (ferrite ratio, graphite ball morphology and quantity) and chemical composition (especially Mn and P content) of your current QT450.

 2. Prioritize process adjustment: Step 1: Limit the Mn content to below 0.15% and control P and S. Step 2: Strengthen incubation, especially ensuring the effective implementation of in flow incubation. 

3: Optimize the composition and adopt a high carbon and low silicon solution. 3. Final guarantee: If the elongation rate still hovers around 18% -20% after process adjustment and cannot stably break through 22%, then introducing ferrite annealing process is an inevitable choice. It can consistently deliver the performance you need. If the tensile strength cannot reach 450 megapascals in the above process, which type of alloy should be used for strength defense? In the QT450 scheme that pursues high elongation (>22%), if the elongation meets the standard and the tensile strength decreases, nickel can be added to adjust the strength. The core function and benefits of adding nickel 1 Solid solution strengthening without significantly damaging plasticity: Nickel element will dissolve into the ferrite matrix to form a solid solution, thereby improving strength without significantly reducing plasticity and toughness. This is fundamentally different from elements such as manganese and phosphorus.

 Effect: When you try to reduce the manganese content and pearlite in order to achieve ultra-high elongation, the tensile strength may slip to the edge of 450MPa. At this point, adding a small amount of nickel can provide a "safety pad" to ensure stable strength and compliance with standards. 

2. Refine the structure and improve uniformity: Nickel can lower the austenite transformation temperature, which helps refine the grain size and microstructure, making the casting structure more uniform, thereby improving both strength and toughness. 

3. Mild pearlite stabilization effect: Nickel also has a tendency to stabilize pearlite, but its effect is far less strong than manganese. By controlling the amount of addition, it is possible to obtain most of the ferrite while utilizing it to form a small amount of fine pearlite for reinforcement. How to scientifically add nickel? Prerequisite: Nickel addition must be carried out after strictly implementing all the basic schemes mentioned above (low Mn, low P/S, strong incubation, etc.). We cannot expect to use nickel to compensate for the shortcomings of basic processes. 1. Addition amount and expected effect: Low nickel solution (0.5% -1.0%): Objective: To provide moderate solid solution strengthening as a "safety net" for strength. Effect: On almost all ferritic substrates, the tensile strength can be increased by about 20-40 MPa. This is sufficient to steadily increase the strength at critical values (such as 430-440 MPa) to above 450 MPa, while having minimal impact on elongation (possibly only reducing by 1-2%), and still easily maintaining above 22%. Medium nickel scheme (1.0% -2.0%): Objective: While providing reinforcement, it may introduce a small amount (<10%) of pearlite. Effect: The strength improvement will be more significant (up to 50 MPa or more), but the elongation will decrease slightly. Careful control is required and adjustments should be made through heat treatment. 2. Collaboration with heat treatment: As cast solution: If you want to achieve high strength and high plasticity in the as cast state without heat treatment, low nickel addition (such as 0.5%) is a very sophisticated strategy. Heat treatment plan: If you have already planned ferrite annealing, the significance of adding nickel needs to be re evaluated. Annealing will eliminate pearlite, and the solid solution strengthening effect of nickel becomes dominant. At this point, low nickel addition can still provide a pure but stronger ferrite matrix after annealing. The disadvantages and cost considerations of adding nickel are high: nickel is an expensive alloying element that significantly increases raw material costs. A rigorous cost-benefit analysis must be conducted. Limited effect: Nickel is not a "panacea", it cannot save a poor substrate with poor spheroidization, failed incubation, or high Mn/P content. Possible introduction of uncertainty: Excessive addition of nickel (such as>1.5%) can stabilize too many pearlite, requiring higher annealing temperatures or longer holding times to eliminate, increasing the difficulty and energy consumption of heat treatment, and may ultimately damage the elongation rate. The conclusion and final recommendation consider nickel addition as the 'last fine tuned insurance' rather than the primary means. The performance optimization path should be: 1 First priority (foundation and core): Extreme purification: Reduce Mn to<0.15%, P<0.03%，S<0.012%。 Strong Fertility: Resolutely implement "one-time fertility+flow fertility", with a target graphite ball count of>150/mm ². Composition optimization: Using high carbon equivalent (~4.5%), controlling the final Si at 2.2% -2.5%. 2. Second priority (evaluation and fine-tuning): After strictly implementing the first priority plan, pour test bars and test their performance. If the result shows that the elongation rate far exceeds 22% (such as 25% or more), but the strength fluctuates within the range of 440-450 MPa, it is on the verge of reaching the standard. So decision: At this point, adding around 0.5% nickel is the best choice. It can achieve stable strength at a very low cost (with minimal impact on elongation) and has the highest cost-effectiveness. 3. Third priority (final guarantee): If the performance is still unstable due to casting wall thickness or cooling rate, ferritization annealing is the final and most reliable solution. Under annealing process, even without adding nickel, it is almost always possible to meet the requirements of strength (relying on solid solution strengthening of graphite balls and Si) and ultra-high elongation (relying on pure ferrite) simultaneously. In summary, nickel can be added, but it is a "tonic" rather than a "staple food". In this pursuit of ultimate elongation, low nickel addition (~0.5%) is a clever tool used in the final stage to "precisely maintain strength".