Our company, Aobo Steel, mainly supplies forged tool steel. This article will discuss the problems we faced while forging steel and how to avoid them. Generally, round bar steel that is 70mm or larger is forged. Bars smaller than 70mm are hot-rolled. Forging is used for plates with thicknesses of 30mm or more, and hot-rolling is used for thicknesses below 30mm.
The production process of forged tool steel
Liquid steel is poured into ingot molds and solidified into steel ingots. The ingots for tool steel molds typically weigh 0.5 to 10 tons. When remelting and refining are required, they become the base material ingots for processes like ESR (Electroslag Remelting). Subsequently, they are forged using a forging machine.
The ratio of the initial cross-sectional area of a steel ingot to its cross-sectional area after forging is called the forging ratio. As the forging ratio increases, the solidified microstructure is gradually broken down into finer grains, and small defects are eliminated, thereby improving toughness and ductility. Generally speaking, when the forging ratio exceeds 5, the toughness reaches almost saturation, which is considered the minimum forging ratio.
Defects and Prevention
Ingot Defects
1. Segregation
Segregation occurs when a steel ingot’s chemical composition and impurities are distributed unevenly during solidification. This uneven distribution is caused by selective crystallization, changes in solubility, density differences, and variations in flow rate. Different elements have different solubilities at different temperatures in the solid and liquid phases. Crystallization differences caused by varying temperature gradients, shrinkage during the solidification process, and various chemical reactions can all lead to segregation, resulting in the non-uniform distribution of components in both macro and micro-regions. Segregation can cause non-uniform mechanical properties and cracking defects. Dendritic segregation in steel ingots can be eliminated through forging, recrystallization, high-temperature diffusion, and post-forging heat treatment. However, zonal segregation is difficult to eliminate through heat treatment methods and can only be homogenized through repeated upsetting and drawing deformation processes.
2. Inclusions
Non-metallic compounds that do not dissolve in the metal matrix are called non-metallic inclusions. Common non-metallic inclusions include sulfides, oxides, and silicates. Inclusions can be classified into two types: internal inclusions and external inclusions. Internal inclusions are the products of chemical reactions during smelting and casting, while external inclusions are impurities such as sand, refractory materials, and furnace fragments introduced from the outside during smelting and casting. Their presence hurts both the hot forging process and the quality of the forged part. They disrupt the continuity of the metal, and under stress, stress concentration occurs at the inclusions, which can lead to microscopic cracks and become the source of fatigue failure in the forged part.
3. Gases and Bubbles
Steel liquid dissolves many gases, such as hydrogen, nitrogen, and oxygen. Since their solubility in steel liquid is much higher than that in solid steel, a large amount of gas will inevitably be released during the solidification of the steel ingot, but some will still remain inside or under the skin of the steel ingot, forming bubbles. As long as the bubbles inside the steel ingot are not open or are open but the inner walls are not oxidized, they can be forged and welded, but the subcutaneous bubbles can easily cause cracks. The common residual gases in steel ingots are oxygen, nitrogen, and hydrogen. Among them, oxygen and nitrogen eventually exist as oxides and nitrides in the steel ingot, forming inclusions in the steel ingot. Hydrogen is the most harmful gas in steel.
4. Shrinkage Cavity and Porosity
During the solidification process of a steel ingot, physical shrinkage occurs. If no liquid steel is replenished, voids will form in certain areas of the ingot. A shrinkage cavity forms at the axial center of the ingot head, where solidification is the latest. This defect is unavoidable due to the lack of liquid steel to fill the void. The size and location of the shrinkage cavity are related to the ingot mold structure and pouring process. If the ingot mold is inappropriate or the top insulation is poor, a secondary shrinkage cavity (pipe) may extend into the ingot body. Generally, the shrinkage cavity and the riser are cut off during forging. Otherwise, the shrinkage cavity cannot be welded due to forging, resulting in internal cracks and forging scrap.
Porosity is caused by the intergranular voids formed by the final solidification shrinkage of the intergranular liquid steel and the micropores formed by the gas precipitation during the solidification of the liquid steel. These pores become larger porosities at the segregation area and smaller pinholes at the dendrite. Porosity reduces the density of the ingot structure, destroys the continuity of the metal, and affects the mechanical properties of the forging. Therefore, large deformation is required during forging to forge through the ingot and eliminate the porosity.
Improper Forging
1. Forging cracking caused by improper heating
- Overheating or uneven heating causes the forging to overheat or burn locally or as a whole, causing the grains to coarsen and grain boundaries to oxidize or melt, resulting in the forging cracking or surface cracking;
- If the heating rate is too fast, the temperature difference between the surface and the interior of the forging is too large, resulting in a large thermal stress and causing cracking;
- Suppose the heating temperature is too low or the holding time is too short. In that case, the temperature difference between the inside and outside is large, resulting in large thermal stress. This stress leads to central or transverse cracking on the edges or flat surfaces of the forging due to the large deformation resistance and reduced plasticity.
2. Cracks Caused by Improper Forging Deformation
Suppose the hammering force is too strong, the deformation amount is too large in a single stroke, and the deformation speed is too fast. In that case, the internal tensile stress will increase due to the large difference in deformation between the surface and the core of the forging. This is prone to the formation of cross cracks in the interior or front and rear end faces. Large and excessive deformation can cause the core temperature to rise and overheat, resulting in cracking.
3. The Impact of Improper Final Forging Temperature on Forging Quality
The final forging temperature directly impacts the quality of die forgings. If the final forging temperature is too high, the grains will continue to grow during the cooling process, thereby reducing the mechanical properties of the steel. If the final forging temperature is too low, under the condition of poor low-temperature plasticity, excessive hammer blows can cause immediate forging cracks. Generally, the final forging temperature should be slightly higher than the temperature of Ar or Ar to ensure that the forging takes place in a single-phase region with relatively uniform plasticity and stress state.
4. Impact of Improper Cooling on Forged Part Quality
Certain cold work tool steels, such as D3 | 1.2080 | SKD11, have high alloy content and good permeability. They can undergo martensitic transformation upon air cooling from high temperatures. Under the combined action of internal and residual stress from deformation, these steels are prone to longitudinal cracking if not slowly cooled after forging. For such tool steels, slow cooling methods such as sand cooling, ash cooling, furnace cooling, or immediate annealing after forging are necessary.
5. White spots appearing after forging
This primarily occurs in medium-carbon low-alloy large-section modules, such as 1.2714 tool steel, and sometimes also in low-carbon medium-alloy precipitation-hardening steels. The main reason is excessive hydrogen in the steel, coupled with rapid cooling at low temperatures (150-250℃) after forging, leading to brittle fracture and the formation of white spots (cracks) in the steel. The presence of white spots reduces the mechanical properties of the steel and can lead to quenching cracks. If white spots are detected, the forging ratio should be increased, and the large section should be reforged into a smaller section to try to weld the cracks together. Otherwise, the steel with white spots should be rejected.
About forging tool steels from Aobo Steel
We have more than twenty years of experience in forging tool steel production, located in Huangshi, the main output of die steel in China. We have rich experience in producing common mold steel, such as D2 tool steel | 1.2379 | SKD11 D3 tool steel | 1.2080 | SKD1 A2 tool steel | 1.2363 | SKD12 H11 tool steel | 1.2343 | SKD6 H13 tool steel | 1.2344 | SKD61 and so on. We also have R&D capability for customized tool steel.
English 
Portuguese