A Comprehensive Analysis of Materials for Pushing Bending Machines: From Tube Billets to Mold Selection Systems
In industrial pipeline systems, pushing bending heads, as key pipe fittings for changing the flow direction of media, directly affect the safety and service life of the entire pipeline system. The core factor determining the quality of the bending heads is the material selection involved in the pushing process. As a specialized equipment for producing such pipe fittings, the material of the tube billets processed by the pushing bending machine and the materials of its key components together form a complete material system. This article starts with the classification of tube billet materials for pushing bending heads, systematically sorts out different categories such as carbon steel, alloy steel, stainless steel, non-ferrous metals, and special materials, and extends to the material selection requirements for the pushing machine molds and core components, providing a comprehensive material selection reference for relevant practitioners. I. Definition of Material Concepts in the Pushing Process Before discussing the material classification of pushing bending machines, it is necessary to clarify a basic concept: the "material" in the pushing process includes two dimensions. The first dimension is the material of the tube billet being processed, which determines the material properties of the finished bending head, including its mechanical properties, corrosion resistance, applicable temperature range, and application scenarios. The second dimension is the material of the pushing machine molds and core components, which need to have sufficient high-temperature strength, wear resistance, and thermal fatigue resistance to ensure the stability of the pushing process and the dimensional accuracy of the product. The pushing bending process usually uses medium-frequency induction heating or furnace heating to heat the tube billet to a thermoplastic state, and then, under the action of hydraulic thrust, it is expanded and bent along the mandrel and the bending head mold. During this process, the material of the tube billet and the mold need to be matched. It is necessary to ensure that the tube billet has good fluidity and formability at high temperatures, and at the same time, ensure that the mold maintains dimensional stability and sufficient service life during repeated thermal cycles. The following will discuss in detail from the two aspects of tube billet material and mold material. II. Classification of Tube Billet Materials for Pushing Bending Heads (1) Carbon Steel Material Series Carbon steel is the most basic and widely used material category for pushing bending heads, with the main characteristics of moderate cost, mature process, and stable mechanical properties. According to the carbon content and sulfur and phosphorus impurity content, carbon steel can be further divided into ordinary carbon structural steel and high-quality carbon structural steel. Ordinary carbon structural steel, such as the Q235 series, is often used in low-pressure pipeline systems with low mechanical performance requirements. The pushing bending heads made of this material have relatively loose control over heating temperature, and the deformation resistance during the pushing process is small, with less wear on the mold. However, due to the wide range of carbon content fluctuations and slightly poor microstructure uniformity, special attention should be paid to the uniformity of heating temperature and the control of pushing speed when pushing large-diameter or thick-walled bending heads to avoid uneven wall thickness or surface microcracks. High-quality carbon structural steel, such as 20# steel and 10# steel, is the most widely used material category for pushing bending heads. 20# steel has good comprehensive mechanical properties and welding performance, and its yield strength and tensile strength can meet the conventional requirements of medium and low-pressure pipelines. In the pushing process, the heating temperature of 20# steel is usually controlled between 850°C and 1050°C. Within this temperature range, the material is in the austenite phase region, with good plasticity and moderate deformation resistance, making it easy to form. After the pushing process, the bending head can be subjected to normalizing or annealing treatment to obtain a uniform ferrite plus pearlite structure, eliminate residual stress, and ensure dimensional stability. For higher requirements of carbon steel bending heads, such as those involving low-temperature environments or higher pressure grades, low-alloy high-strength steels such as 16Mn (Q355B) are selected. These materials add a small amount of manganese to carbon steel, improving the strength and toughness of the material through solid solution strengthening, and exhibit better thermoplasticity in the pushing process, making them suitable for the production of large-diameter and thick-walled bending heads. (II) Alloy Steel Material Series Alloy steel is a type of steel formed by adding alloy elements such as chromium, molybdenum, vanadium, and nickel to carbon steel. Its prominent features include higher high-temperature strength, oxidation resistance, or low-temperature toughness. In the field of push-bending elbows, alloy steel materials are mainly used in high-temperature and high-pressure pipelines, boiler pipelines, and petrochemical equipment. Alloy steel series represented by chromium-molybdenum steel, such as 12Cr1MoVG, 15CrMoG, and 12Cr2MoG, are indispensable materials in power station boilers and thermal pipelines. The push-bending process of these materials requires extremely strict temperature control, with the heating temperature usually precisely controlled within the range of 950°C to 1100°C. The heating rate should not be too fast to prevent alloy element segregation and coarsening of the microstructure. Due to the strong deformation resistance of chromium-molybdenum steel at high temperatures, the required pushing force is significantly higher than that of carbon steel, which poses higher requirements for the stability of the hydraulic system and medium-frequency heating system of the push-bending machine. After the push-bending process is completed, chromium-molybdenum steel elbows usually need to undergo normalizing and tempering treatment to obtain tempered bainite or tempered sorbite microstructure, which is crucial for ensuring the material's high-temperature endurance strength and creep resistance. It is particularly important to note that chromium-molybdenum steel is sensitive to cooling rates, and the cooling method after push-bending needs to be strictly controlled to prevent the formation of martensite, which can lead to increased hardness and reduced toughness. Alloy steels with higher nickel and chromium content, such as austenitic heat-resistant steels, exhibit completely different characteristics in the push-bending process. These materials maintain good toughness and oxidation resistance at high temperatures but have a larger coefficient of thermal expansion. The cooling shrinkage after push-bending needs to be precisely calculated to avoid affecting the angle and dimensional accuracy of the elbow. (III) Stainless Steel Material Series Stainless steel elbows are widely used in food, pharmaceutical, chemical, and marine engineering fields due to their excellent corrosion resistance. According to the different types of microstructure, the commonly used stainless steel materials for push-bending elbows can be classified into three major categories: austenitic stainless steel, ferritic stainless steel, and duplex stainless steel. Austenitic stainless steels such as 304, 304L, 316, and 316L are the most widely used stainless steel category in push-bending elbows. These materials have a face-centered cubic crystal structure, are non-magnetic, and have excellent toughness and plasticity. They exhibit excellent formability in the push-bending process. The push-bending temperature of austenitic stainless steel is usually controlled between 1050°C and 1150°C, and special attention should be paid to preventing carburization and surface oxidation during heating. Since austenitic stainless steel does not undergo phase transformation during heating, its tendency for grain growth is relatively obvious, so the heating time needs to be strictly controlled to avoid grain coarsening and a decrease in mechanical properties. During the push-bending process, the work hardening phenomenon of austenitic stainless steel is relatively significant. Although this problem is somewhat alleviated at high temperatures, it is still necessary to pay attention to the matching of the pushing speed and heating power to prevent local hardening or cracking due to excessive deformation speed. After push-bending, austenitic stainless steel elbows usually need to undergo solution treatment, which involves heating the elbow to above 1050°C and rapid cooling to eliminate processing stress and restore corrosion resistance. Ferritic stainless steels such as 430 and 444 have a body-centered cubic crystal structure and relatively low cost, but their high-temperature plasticity is slightly inferior to that of austenitic stainless steel. The push-bending temperature range for these materials is relatively narrow, usually between 850°C and 950°C. Excessive temperature can lead to coarse grains and reduced toughness, while too low a temperature increases deformation resistance. When push-bending ferritic stainless steel elbows, the surface finish of the die and lubrication conditions are required to be high to reduce friction resistance and prevent surface scratches. Duplex stainless steels such as 2205 and 2507 have a dual-phase microstructure of austenite and ferrite, offering the dual advantages of high strength and high corrosion resistance. The extrusion process of this type of material is quite challenging due to its high sensitivity to temperature in terms of thermoplasticity. The proportion of the two phases changes with temperature, and the extrusion temperature needs to be precisely controlled within the appropriate range of the two-phase zone. Additionally, duplex stainless steel is prone to the formation of harmful phases at high temperatures, so both heating and cooling rates need to be reasonably controlled to ensure that the extruded elbows have the desired two-phase ratio and mechanical properties. (4) Non-ferrous metals and special materials series Apart from steel materials, the extrusion of elbows also involves non-ferrous metals such as copper alloys, nickel-based alloys, and titanium alloys, as well as special materials. The extrusion processes of these materials each have their own characteristics, and the requirements for equipment and molds are also more specific. Copper alloy elbows, such as brass and bronze, are mainly used in water supply, heating, and refrigeration systems. Copper alloys have relatively low melting points and high thermal conductivity. The heating temperature during extrusion is usually controlled between 600°C and 800°C. Due to the tendency of copper alloys to oxidize at high temperatures, protective atmospheres or rapid forming methods are needed during the extrusion process to minimize the formation of oxide scales. Copper alloys have good fluidity and low forming pressure, but they cause relatively less wear on molds. Nickel-based alloys such as Inconel 600, Inconel 625, and Hastelloy C276 are high-end materials for extruded elbows, mainly used in highly corrosive environments or ultra-high-temperature conditions. The extrusion of nickel-based alloys is extremely difficult. Their characteristics include high high-temperature strength, high deformation resistance, and a significant tendency towards work hardening. When extruding nickel-based alloy elbows, high-power heating systems and high-pressure hydraulic systems are required. At the same time, the materials for the mandrels and molds need to be of extremely high quality. Usually, molds are made of nickel-based or cobalt-based alloys with excellent heat resistance, and special lubricants are used to ensure the smooth progress of the extrusion process. Titanium alloy elbows are mainly used in marine engineering, aerospace, and chemical industries, and they have the advantages of high specific strength and excellent corrosion resistance. The extrusion process of titanium alloys requires extremely strict temperature control. The heating temperature is usually controlled between 850°C and 950°C. Excessive temperature can lead to oxygen and hydrogen absorption and surface embrittlement, while too low a temperature results in insufficient plasticity. Titanium alloys have strong chemical reactivity at high temperatures and are prone to adhesion with mold materials. Therefore, special protective coatings and lubrication measures are needed during the extrusion process, and the mold materials should be selected from alloys with low affinity for titanium alloys. III. Materials for Extrusion Machine Molds and Core Components The core components of an extrusion machine include elbow molds, mandrels, push plates, guide sleeves, and heating induction coils. The material selection of these components directly affects the stability of the extrusion process and the consistency of product quality. (1) Materials for elbow molds Elbow molds are key tools that directly determine the shape and size of the elbows during the extrusion process. They need to have good high-temperature strength, thermal fatigue resistance, wear resistance, and sufficient toughness. Common materials for elbow molds include cast high-temperature alloys, hot work die steels, and nickel-based alloys. For the extrusion of carbon steel and low-alloy steel elbows in large quantities, hot work die steels such as H13 (4Cr5MoSiV1) and 3Cr2W8V are typically used. After quenching and tempering, these materials can achieve high hardness and good red hardness, maintaining dimensional stability during repeated thermal cycles. H13 die steel has good toughness and strong resistance to thermal fatigue cracks, making it suitable for the extrusion of medium-batch, conventional material elbows. For the extrusion of difficult-to-deform materials such as stainless steel and nickel-based alloys, or for the production of large-diameter, thick-walled elbows, cast high-temperature alloys with better heat resistance, such as K418 and K423, are required. These materials are based on nickel and contain elements such as chromium, cobalt, molybdenum, aluminum, and titanium, achieving excellent high-temperature strength through precipitation hardening. They can maintain high hardness and creep resistance above 900°C. The service life of molds for casting high-temperature alloys is much longer than that of hot work die steel, but the material cost and processing difficulty also increase accordingly. In some high-end applications, nickel-based alloy powder metallurgy materials or cobalt-based alloys are used to make elbow molds. These materials have reached the extreme in terms of high-temperature performance and thermal fatigue resistance, but they are expensive and are usually only used in the production of high-value-added products. (2) Core Rod Materials The core rod is a key component in the pushing process for achieving the expansion and guidance of the elbow. Its shape directly determines the geometry and wall thickness distribution of the inner surface of the elbow. The core rod material needs to have excellent high-temperature strength, oxidation resistance, and anti-deformation ability. For medium and small diameter elbow pushing, common core rod materials are high-temperature alloys such as GH4169 and GH3030. GH4169 is a precipitation-hardening deformable high-temperature alloy based on nickel-chromium, which has high strength and good fatigue resistance below 650°C and is suitable for pushing carbon steel and stainless steel elbows. For higher pushing temperatures and greater forces, GH3039 and GH3128, which are solid solution-strengthened high-temperature alloys, are selected. These materials can maintain sufficient strength at higher temperatures and have excellent oxidation resistance. The surface quality of the core rod directly affects the smoothness of the inner surface of the elbow. Therefore, the core rod material also needs to have good processing performance and surface hardening ability. During the pushing process, the core rod is in direct contact with the inner surface of the high-temperature tube blank and generates relative sliding, requiring the use of special lubricants to reduce friction and wear. (3) Materials for Other Key Components The push plate is the component that applies the pushing force and directly contacts the end of the tube blank, needing to withstand huge axial pressure. The material for the push plate is usually hot work die steel or cast steel, which is quenched and tempered to obtain good comprehensive mechanical properties. The end face of the push plate in contact with the tube blank needs to be kept flat, and when necessary, heat-resistant hard alloys can be embedded to extend its service life. The guide sleeve is used to maintain the straight-line movement of the tube blank during the pushing process and prevent deviation. The material for the guide sleeve is usually wear-resistant cast iron or copper alloy, and its inner surface needs to be machined with oil grooves to ensure lubrication. The induction heating coil is the core component of the medium-frequency heating system, made of high-purity oxygen-free copper tubes that are bent into spiral or special-shaped coils. Oxygen-free copper has high electrical and thermal conductivity, which can efficiently convert medium-frequency electrical energy into heat energy. At the same time, the heat generated is carried away by internal circulating cooling water to ensure long-term stable operation. IV. Comprehensive Impact of Material Selection on Pushing Process The choice of materials not only determines the performance of the elbow products but also has a profound impact on the parameter setting of the pushing process, equipment selection, and cost control. Different materials of tube blanks have different requirements in terms of heating temperature, pushing speed, and cooling methods. Carbon steel has a wider process window and relatively lower pushing difficulty; alloy steel and austenitic stainless steel have higher requirements for temperature control accuracy; duplex stainless steel and nickel-based alloys have strict requirements for heating speed, cooling speed, and atmosphere control. Pushing machine operators need to formulate corresponding process regulations based on the characteristics of different materials and strictly follow them during production. The compatibility of materials with molds is equally important. When pushing materials with high deformation resistance, molds with higher high-temperature strength need to be selected, and the hydraulic system of the pushing machine should have sufficient pushing force reserve. For materials prone to surface scratches or adhesion, special coating treatment or materials with better anti-adhesion properties should be used on the mold surface. From a cost perspective, the selection of materials needs to take into account both the use requirements and the feasibility and economy of the process. Under the premise of ensuring quality, reasonable selection of tube blank materials and mold materials, and optimization of process parameters can effectively improve production efficiency, reduce mold wear, and achieve the maximization of comprehensive benefits. V. Conclusion The material system involved in the push-bending machine is a systematic project covering both the tube blank materials and the tooling materials. From carbon steel, alloy steel, and stainless steel to nickel-based alloys and titanium alloys, each material has its unique processing characteristics and application scenarios; from hot work die steel, cast high-temperature alloys to nickel-based alloys, each mold material needs to be matched with the processing object. A thorough understanding of the classification, properties, and processing requirements of these materials is the foundation for ensuring the quality of push-bent elbows, improving production efficiency, and reducing manufacturing costs. As industrial pipeline systems evolve towards higher pressure, stronger corrosion resistance, and wider temperature ranges, the materials for push-bent elbows are also showing a trend of diversification and high performance. At the same time, the materials and manufacturing processes of push-bending machine molds are also constantly advancing, with the application of ceramic materials and surface engineering technologies. |
