STEEL FORGING VS. CASTING

BASIC DEFINITIONS

Steel casting and forging are two fundamental manufacturing processes used to form steel into desired products, each with distinct characteristics, advantages, and applications. Understanding the differences between these methods is crucial for choosing the most suitable approach for specific industrial needs.

Steel Casting

Steel casting involves melting steel into a liquid state and pouring it into a mold, where it solidifies into a specific shape. This process is advantageous for creating complex shapes and components that would be challenging to manufacture through other methods. Cast steel products can range from simple parts like gears and valves to intricate designs such as components for heavy machinery and automotive assemblies.

Steel casting finds applications in industries where complex shapes or specialized materials are required, including automotive, mining, aerospace, and construction.

Steel Forging

Forging involves heating solid steel blocks (ingots or billets) and then deforming them under high pressure or impact to achieve the desired shape. This process can be performed at various temperatures, leading to classifications such as cold forging, warm forging, and hot forging.

Steel forging is widely used in industries requiring high-strength components, such as automotive and aerospace for parts like gears, shafts, levers, and critical fasteners (including valves).

Both manufacturing processes require the application of high temperatures to steel raw materials (to liquefy or make it malleable) and the execution of CNC machining work at the end of the process to obtain the final product.

Final products may also undergo surface finish treatment, such as painting, powder coating, polishing, various types of coating (for example zinc plating) and wear protection/hardening (application of tungsten carbide overlay).

Last but not least, cast and forged parts may be assembled, welded, brazed, and hard-faced before being shipped as final products.

The products resulting from casting and forging processes have different properties in terms of surface porosity (generally better for forged vs. cast products), grain structure (finer for forged products), tensile strength (generally superior for forged products), and fatigue resistance.

These alternative manufacturing processes are therefore used (and suited for) different circumstances and applications.

CHOOSING BETWEEN CAST & FORGED PRODUCTS

The decision to use casting or forging depends on several factors:

• Design Complexity: Casting is preferred for more complex shapes while forging is ideal for simpler, high-strength parts.
• Material Properties: If the application requires specific alloy properties, casting may offer more flexibility. For applications where material strength and integrity are paramount, forging is often the better choice.
• Production Volume: Forging can be more cost effective for large production runs of simpler shapes, whereas casting can be more economical for small to medium batches, especially of complex parts.
• Cost Considerations: Initial setup costs for casting molds can be high, but per-unit costs decrease with volume. Forging requires significant investment in tooling and machinery, especially for high-volume production.

The casting process is generally preferred for:

• parts and components that would be too complex or expensive to manufacture by steel forging (for example: large valve bodies);
• parts that have internal cavities;
• large-sized parts (there are virtually no size limits in terms of the weight of the parts that can be produced with the casting process);
• parts in special alloys (some specific alloys are more difficult to forge than cast, for example, those with a high content of Nickel and Moly, which have considerable resistance to mechanical forces);
• parts requiring mass production and small lots.

The forging process is preferred for:

• parts requiring extremely high strength, toughness, and resistance (indeed, during the forging process the steel grain structure gets modified to conform to the shape of the final product – with high uniformity of composition and metallurgical recrystallization);
• parts that have to withstand stronger impacts and mechanical forces;
• parts where porosity, the risk of a gas void, pockets, and the possible formation of cavities (even micro-granular) are not acceptable;
• production of mechanically strong parts without using expensive alloys;
• parts that require high wear resistance;
• parts subject to high loads and stress;
• high-end applications when the integrity and the quality of the part are the main objectives in the production process, rather than time and cost.

 

The evolution of casting technologies has reduced the gap between the physical properties of cast vs. forged products making modern cast products very competitive in terms of quality, strength, and wear resistance: however, in many fields, steel forging remains, still, the preferred manufacturing option (example: small sized valves, i.e. forged valves, or high-pressure valves).

Read about forging steel on Wikipedia.

FIELDS OF APPLICATION

Steel casting and forging are used to produce products and parts for a multitude of sectors, including but not limited to:

• petrochemical plants (for example forged valves, forged fittings, flanges, etc)
• power generation and waste processing
• mining and mineral processing
• agriculture and livestock handling
• water treatment
• aeronautics
• automobile industry (pulleys and gear wheels)
• materials handling
• brickworks
• asphalt plants
• stormwater parts
• rendering plants
• railways

STEEL CASTING

DEFINITION

Steel casting is a manufacturing process that involves melting steel until it becomes liquid, then pouring the molten steel into a mold where it solidifies into the desired shape. This method allows for the production of parts with complex geometries and detailed features that might be challenging to achieve through other manufacturing processes.

The molds used can be made from a variety of materials, including sand, metal, or ceramics, depending on the precision, surface finish, and reuse requirements of the casting. Steel castings are utilized across a broad range of industries, including automotive, aerospace, construction, and energy, due to their versatility in shape and size, and the ability to tailor the material properties through the selection of specific steel alloys and post-casting treatments such as heat treatment.

This process is particularly valuable for producing components that require high strength, durability, and resistance to wear and corrosion.

STEEL CASTING PROCESS

The steel casting manufacturing process involves several key steps that transform steel into precisely shaped components. This process is known for its ability to produce complex shapes and sizes, offering flexibility in design and material properties.

Here’s an overview of the main stages in the steel casting manufacturing process (in general terms, variations may exist based on the specific type of casting process adopted):

1. Pattern Making

The first step is creating a pattern that represents the shape of the final casting. Patterns can be made from various materials such as wood, plastic, or metal, depending on the complexity of the design and the casting quantity. The pattern is a replica of the final part but slightly larger to account for the shrinkage of the metal during cooling.

2. Mold Making

Using the pattern, a mold is made by packing a molding material (often sand mixed with binders) around the pattern in a molding box. When the pattern is removed, it leaves a cavity in the shape of the part to be cast. For complex parts, the mold consists of two halves (cope and drag) that are assembled before pouring. Cores may be used to form internal cavities within the casting.

3. Melting and Pouring

Steel is melted in a furnace at temperatures exceeding 1600°C (2900°F). The composition of the steel alloy is carefully controlled to meet the specific properties required for the casting. Once the steel is melted and any required alloying elements are added, the molten steel is poured into the prepared mold. The pouring process must be carefully controlled to ensure the mold is filled completely and to avoid defects.

4. Solidification and Cooling

After pouring, the steel begins to cool and solidify within the mold. Cooling rates can affect the microstructure and properties of the cast part. In some cases, directional solidification is used to control the way the metal cools, minimizing internal stresses and defects.

5. Mold Removal

Once the steel has fully solidified and cooled, the mold material is broken away to reveal the cast part. Sand molds are typically destroyed to remove the casting, while metal molds (used in permanent mold casting processes) are opened and reused.

6. Cleaning and Finishing

The casting is cleaned to remove any residual mold material and inspected for defects. Excess material from the pouring process, such as gates and runners, is removed. The casting may undergo various finishing processes, including grinding, machining, and heat treatment, to achieve the desired surface finish and mechanical properties.

7. Inspection and Quality Control

The final step involves inspecting the casting for defects and ensuring it meets the specified dimensions and material properties. Non-destructive testing methods such as X-ray or ultrasonic testing may be used to detect internal defects.

ADVANTAGES/DISADVANTAGES OF CAST STEEL

Advantages

Cast steel offers a unique set of advantages that make it a preferred choice for a wide range of applications across various industries. These benefits stem from the versatility of the casting process as well as the intrinsic properties of steel. Here are some of the key advantages of cast steel:

1. Design Flexibility

Cast steel allows for the production of complex shapes and intricate designs that are difficult or impossible to achieve with other manufacturing processes. This flexibility enables designers and engineers to create components with optimized geometries for specific applications, incorporating features like internal cavities, contours, and complex external shapes without the need for assembly or welding of multiple parts.

2. Material Properties

Steel casting can utilize a wide range of steel alloys, offering the ability to tailor the material properties to specific application requirements. This includes adjusting the alloy composition to enhance strength, ductility, wear resistance, impact resistance, and corrosion resistance. Additionally, post-casting heat treatments can further modify the mechanical properties, providing additional customization options to meet the demands of the application.

3. Large Component Production

The steel casting process is well-suited for producing large components that would be challenging or cost-prohibitive to manufacture through forging or machining from a solid block of steel. This capability is particularly valuable in industries such as mining, construction, maritime, and energy, where large, durable components are essential.

4. Cost-Effectiveness

For small to medium production volumes, casting can be more cost-effective than other manufacturing methods, especially for complex shapes that would require extensive machining or assembly of multiple parts. The ability to produce near-net-shape components reduces the need for additional processing and material waste, further contributing to cost savings.

5. Surface Finish and Detail

Cast steel can achieve a good surface finish and a high level of detail directly from the mold, reducing the need for additional finishing processes. This is beneficial for parts that require precise dimensions or aesthetic considerations.

6. Reliability

Steel castings undergo stringent quality control and testing procedures to ensure they meet the required specifications and standards. Non-destructive testing (NDT) techniques, such as x-ray, ultrasonic, and magnetic particle inspection, help detect any internal or surface defects, ensuring the reliability and performance of the cast component in its intended application.

7. Speed to Market

The casting process can be relatively quick from design to production, especially for prototypes or limited production runs. This speed can significantly reduce the time to market for new products or components, providing a competitive advantage.

In conclusion, the combination of design flexibility, customizable material properties, and cost-effectiveness makes cast steel a versatile and valuable option for a wide range of industrial applications. From heavy machinery and automotive components to artistic and architectural elements, the advantages of cast steel enable engineers and designers to meet complex challenges and deliver innovative solutions.

Disadvantages

The manufacturing and utilization of cast steel components offer distinct advantages but also come with certain disadvantages. Understanding these can help in making informed decisions about when and how to use cast steel in various applications:

1. Cost

• Initial Costs: The production of cast steel components can involve high initial costs due to the need for specialized molds, which can be expensive to manufacture, especially for intricate designs or low production volumes. This can make cast steel less economical for small batches.
• Material Costs: Steel, especially alloyed varieties used in casting for enhanced properties, can be more expensive than other metals, adding to the overall cost of the casting process.

2. Surface Finish and Tolerances

• Surface Finish: Cast steel might have a rougher surface finish compared to other manufacturing processes like forging or machining. This can necessitate additional finishing processes, increasing production time and costs.
• Dimensional Tolerances: Achieving tight dimensional tolerances can be more challenging with cast steel, especially for complex shapes or large components. This might require further machining or processing to meet precise specifications.

3. Porosity and Defects

• Porosity: The casting process can introduce porosity (tiny holes or voids) in the steel, which can affect the mechanical strength and integrity of the part. Porosity is particularly concerning in applications where pressure integrity or structural strength is critical.
• Defects: Castings are susceptible to various defects such as shrinkage cavities, cold shuts, and inclusions. These defects not only compromise the structural integrity and mechanical properties of the part but also require rigorous inspection and quality control measures to detect and mitigate.

4. Material Limitations

• Alloy Restrictions: While casting allows for the use of a wide range of alloys, some compositions might not be suitable for casting due to their melting characteristics or reactions with mold materials. This can limit the choice of materials for specific applications.
• Size and Weight Limitations: There are practical limits to the size and weight of cast steel components, dictated by the capacity of foundry equipment and the challenges of managing heat dissipation and material properties uniformly across large castings.

5. Environmental Considerations

• Energy Consumption: The melting and casting of steel require significant amounts of energy, contributing to the environmental footprint of cast steel products.
• Waste Production: The process can generate waste, including used molds and cores, excess metal from gates and risers, and defective castings, which need to be managed and recycled when possible.

TYPES OF STEEL CASTING PROCESSES

Steel casting processes can be categorized based on the type of mold used, the method of mold filling, and the handling of the molten steel. Each process has its unique advantages and is suited to specific applications. Here’s an overview of the primary types of steel casting processes:

Sand Casting

Sand casting is the oldest casting method and consists of pouring liquid metal into binders that resist the molten metal (such as clay bonded/green sand hard bonded/resin, thermosetting resin sand, and shell).

• Description: This is the most common and versatile method, using sand molds to create steel parts. The sand can be shaped into complex forms, making it suitable for a wide range of components.
• Applications: Used for large parts and small to medium production runs, including automotive frames, heavy machinery components, and decorative pieces.

Investment Casting (Lost Wax Casting)

This term refers to precision molding executed by injecting the liquid metal into a metal die and a ceramic coating. The mold material can be hard wax, lost wax, lost foam, and similar. Investment casting is the preferred method for manufacturing parts with a high number of details, and small parts that cannot afford the costs of traditional sand casting.

• Description: A precision casting process where a wax model is coated with refractory material to create a mold. After the wax is melted out, molten steel is poured into the cavity. This method provides excellent surface finish and dimensional accuracy.
• Applications: Ideal for small, complex components with tight tolerances, such as turbine blades, medical equipment, and firearm components.

Centrifugal Casting

• Description: In this process, molten steel is poured into a rotating mold, creating parts with cylindrical shapes. The centrifugal force pushes the molten steel against the mold walls, producing a dense and uniform casting.
• Applications: Used for components requiring high material integrity, such as pipes, cylinders, and rings.

Continuous Casting

• Description: A highly efficient process where molten steel is solidified in a continuous strand through a mold. The solidified steel is then cut into desired lengths. This method significantly reduces the need for further processing.
• Applications: Primarily used for producing basic shapes like billets, blooms, and slabs, which are further processed into various steel products.

Shell Molding

• Description: Similar to sand casting, but uses a resin-coated sand that bonds to form a shell around the pattern. This method offers a better surface finish and more precise dimensions than traditional sand casting.
• Applications: Suitable for medium to high-volume production of small to medium-sized parts, such as valve components and gear housings.

Vacuum Casting

• Description: A process where molten steel is poured into a mold under a vacuum. This reduces the occurrence of defects and improves the quality of the casting by minimizing turbulence and gas entrapment.
• Applications: Used for components that require high integrity and are free of pores and voids, such as in the aerospace and power generation industries.

Die Casting

• Note: While die casting is predominantly used with non-ferrous metals due to the high melting point of steel, there are specialized high-pressure die-casting processes capable of handling steel alloys.
• Applications: Suitable for high-volume production of small, precise components that require a good surface finish.

Each steel casting process has its ideal applications based on the required part size, complexity, production volume, and material properties. Selecting the appropriate casting method is crucial for achieving the desired part performance and cost-efficiency. Sand casting is used for large parts, investment casting for small parts up to 100 kilograms and 1,5 meters of max. length.

STEEL FORGING

DEFINITION

Steel forging is a manufacturing process involving the shaping of metal using localized compressive forces. In steel forging, steel is heated to a high temperature, making it pliable, and then it is hammered, pressed, or rolled into a desired shape.

This process can be performed at various temperatures, leading to different classifications such as hot forging, warm forging, and cold forging, each offering distinct advantages in terms of the material’s ductility and finished properties.

Steel forging appeared in China in the ancient ages to produce various types of metal products.

While the methods and equipment for creating forged components have evolved since those old days—from traditional anvils, hammers, and manual labor to modern automated machinery like hydraulic presses—the fundamental process of steel forging remains consistent. It involves heating solid steel blocks and then shaping them into finished products through the application of mechanical forces, such as hammering.

FORGING PROCESS

The steel forging process is a critical manufacturing technique used to shape metal into desired forms through the application of force and heat. It involves several key steps that transform a piece of steel into a strong, durable component with specific properties. Here’s a detailed look at the steel forging process:

1. Material Selection

The process begins with selecting the appropriate type of steel for the intended application. The choice depends on the required mechanical properties, such as strength, ductility, and resistance to wear and corrosion. Commonly used steel grades in forging include carbon steel, alloy steel, stainless steel, and tool steel.

2. Heating

The selected steel is heated to a high temperature, typically between 950°C and 1250°C (1742°F and 2282°F), although the exact temperature depends on the type of steel and the specific forging process being used. Heating the steel makes it more pliable and easier to deform. This step is crucial for hot forging, while warm forging and cold forging require different temperature ranges.

3. Forging

Once the steel is at the appropriate temperature, it is forged using one of several techniques:

• Hammer Forging (Drop Forging): The steel is placed on an anvil and struck repeatedly with a hammer, either manually or mechanically. This method is effective for creating a wide variety of shapes with good precision.
• Press Forging: The steel is compressed between two dies using a hydraulic or mechanical press, applying pressure gradually rather than through successive impacts. Press forging allows for better control over the deformation process and is suitable for large-scale production.
• Rolled Forging: The heated steel is passed through two rolls that decrease its thickness and increase its length. Rolled forging is commonly used for producing long, cylindrical shapes like shafts.

4. Shaping

During the forging process, the steel’s internal grain structure deforms to conform to the shape of the part. This alignment enhances the mechanical properties of the steel, such as strength and toughness. The steel can be shaped into nearly any form, from simple flat sheets to complex geometrical components.

5. Trimming

Excess material, known as flash, is often produced during the forging process, especially in die forging. This excess material is trimmed off while the steel is still hot or after it cools down, depending on the specific process and material properties required.

6. Heat Treatment

After forging, the steel components may undergo various heat treatment processes such as annealing, normalizing, quenching, and tempering. These treatments are used to adjust the steel’s properties, such as increasing hardness, improving ductility, or relieving internal stresses induced during forging.

7. Finishing

The last step involves finishing the forged steel component to achieve the desired surface finish and dimensional accuracy. This may include machining, grinding, and polishing, depending on the application requirements.

8. Inspection and Testing

Finally, the forged steel components are inspected and tested to ensure they meet the required specifications and quality standards. This can involve dimensional checks, visual inspection, and various non-destructive testing methods, such as ultrasonic or magnetic particle inspection.

Upon completion of the process, the resulting products possess remarkable strength, impact toughness, and resistance to wear. This is attributed to the metallurgical changes, specifically recrystallization and grain refinement, that occur due to the thermal and mechanical treatments applied during the process.

The video below shows how steel forging works:

ADVANTAGES/DISADVANTAGES OF FORGED STEEL

Forged steel, known for its strength and reliability, is a preferred material for many industrial applications. However, like any manufacturing process, steel forging has its set of advantages and disadvantages. Understanding these can help in making informed decisions about its use in specific applications.

Advantages Of Forged Steel

1. Superior Strength and Toughness

Forging refines the grain structure of the steel, aligning it with the shape of the product. This enhances the mechanical properties of the steel, making forged parts stronger and more resistant to impact and fatigue compared to cast or machined parts.

2. Improved Reliability

The forging process produces parts with consistent material properties and without internal voids, cracks, or other defects that could weaken the part. This uniformity ensures high reliability and performance, especially in critical applications.

3. Reduced Material Waste

Modern forging techniques are highly efficient, often resulting in less material waste compared to other manufacturing processes like casting or machining. This efficiency can lead to cost savings, especially in high-volume production.

4. Versatility in Materials

Steel forging can be performed with a wide range of alloys, allowing for the production of parts with specific properties tailored to the application, including high-temperature resistance, corrosion resistance, and wear resistance.

5. Cost-Effective for Large Production Runs

Although the initial setup and tooling costs can be high, forging can be more cost-effective than other processes for large production runs due to its material efficiency and the reduced need for secondary processing.

Disadvantages Of Forged Steel

1. Higher Initial Costs

The initial costs for setup and tooling in forging can be significant, especially for complex shapes. This can make forging less economical for small production runs compared to other manufacturing processes.

2. Limitations on Shape Complexity

While forging can produce a wide range of shapes, extremely complex or intricate designs may be challenging to achieve through forging alone and might require additional machining or processing.

3. Size Limitations

The size of forgings is generally limited by the capacity of the forging equipment. Very large components might be difficult or impractical to forge and may require alternative manufacturing methods.

4. Need for Secondary Processing

Depending on the application’s requirements, forged parts may require secondary processes such as machining, heat treatment, or surface finishing to achieve the desired dimensions, properties, or surface quality.

5. Energy Intensive

The forging process, particularly hot forging, requires significant amounts of energy to heat the metal, which can impact the overall environmental footprint of the manufacturing process

TYPES OF STEEL FORGING PROCESSES

Steel forging processes vary in technique and application, each suited to different manufacturing requirements and objectives. These processes are categorized mainly based on the temperature at which the steel is forged (cold, warm, or hot forging) and the method of applying force (hammer or press forging). Here’s a closer look at the primary types of steel forging processes:

1. Hot Forging

• Description: Performed at temperatures typically between 950°C and 1250°C (1742°F to 2282°F), hot forging allows the steel to be shaped easily, as the high temperature increases its plasticity and reduces resistance to deformation.
• Advantages: Enables the forging of complex shapes with less force and energy; reduces strain hardening during the process.
• Applications: Used for parts that require high strength and toughness, such as automotive components, construction equipment, and machinery parts.

2. Cold Forging

• Description: This process is carried out at or near room temperature, offering a cleaner finish and stronger products due to strain hardening.
• Advantages: Produces parts with excellent surface finish and dimensional accuracy; increases the strength of the steel through work hardening.
• Applications: Suitable for smaller products with simpler shapes, like fasteners, screws, and bolts, where high precision and strength are required.

3. Warm Forging

• Description: Warm forging is conducted at temperatures ranging from 650°C to 950°C (1202°F to 1742°F), serving as a middle ground between hot and cold forging.
• Advantages: Balances the benefits of hot and cold forging, allowing for reduced force compared to cold forging and better dimensional accuracy than hot forging.
• Applications: Often used for medium-complexity parts where a balance between strength, precision, and cost is needed.

4. Open-Die Forging

• Description: In open-die forging, the steel is worked between two flat or simply contoured dies that do not enclose the workpiece, allowing the material to flow outward as it is forged.
• Advantages: Highly versatile, capable of producing very large parts, and allows for the adjustment of the workpiece during forging.
• Applications: Ideal for large, simple shapes such as shafts, bars, beams, and plates.

Open die forging involves shaping metal by pressing or hammering it between two dies that don’t entirely enclose the material. The piece of metal is worked upon through successive movements of the dies, which apply force to mold the metal into the desired shape. This technique is demonstrated in the action showcased in the video below:

5. Closed-Die Forging (Impression-Die Forging)

• Description: Steel is compressed between two or more dies that contain a precut profile of the desired part, resulting in a more precise and complex shape.
• Advantages: Produces parts with close tolerances and minimal waste; suitable for high-volume production.
• Applications: Widely used for automotive, aerospace, and industrial components that require detailed features and high strength.

Closed-die forging is a process where the dies close in on the workpiece, either completely enclosing it or partially. The process begins by placing a piece of heated raw material, roughly shaped or sized to match the final product, into the lower die. This process is demonstrated in the following video:

6. Seamless Rolled Ring Forging

• Description: This process involves piercing a hole in a thick, round piece of metal to create a donut shape, which is then rolled and stretched to produce a seamless ring.
• Advantages: Efficiently produces rings with excellent structural integrity and homogeneous characteristics.
• Applications: Commonly used for bearings, flanges, gear rings, and wind turbine components.

Difference Between Open-Die & Close-Die Forging

Open-die and closed-die forging are two fundamental processes used in metalworking to shape metal, especially steel, into desired forms through the application of force. While both processes serve the purpose of forging, they differ significantly in their methodologies, applications, and the types of products they produce. Understanding these differences is crucial for selecting the appropriate forging process for a specific application.

Open-Die Forging Key Features

• Versatility in Shape and Size: Suitable for large or uniquely shaped components that cannot be forged in closed dies.
• Flexibility in Production: Ideal for low-volume production or one-off items due to the adaptability of the process.
• Less Precise Dimensions: The final products may require further machining to achieve the desired dimensions and tolerances.
• Greater Material Handling: Larger workpieces can be accommodated, which is beneficial for creating large industrial components.

Open-Die Forging Applications

Large industrial parts like shafts, beams, and simple shapes that require significant material deformation but not intricate details or precise dimensions.

Close-Die Forging Key Features

• High Precision and Detail: Produces parts with close tolerances and complex geometries without the need for significant post-forging machining.
• Efficiency in High-Volume Production: More suitable for mass production of parts due to the repeatability and speed of the process.
• Limited to Smaller Sizes: Generally used for smaller components due to the constraints of the die sizes and press capacities.
• Reduced Material Waste: The process is designed to closely approximate the final part shape, minimizing excess material.

Close-Die Forging Applications

Automotive, aerospace, and hardware components that require precise dimensions, complex shapes, and high strength, such as gears, levers, and fasteners.

Conclusion

The choice between open-die and closed-die forging depends on the specific requirements of the project, including the size and complexity of the parts, the precision needed, the volume of production, and cost considerations. Open-die forging is favored for its flexibility and capability to produce large parts, whereas closed-die forging is preferred for its efficiency and ability to produce parts with complex shapes and tight tolerances.