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Hardfacing weld(build-up welding or overlaying) is a surface welding process that uses flame, arc and plasma arc to melt base metal to form a wear-resistant, corrosion-resistant and heat-resistant coatings on the surface of the workpiece. Hardfacing has characterized the function of repairing and surface strengthening. With the wide application of wear-resistant materials in industry, hardfacing has become an important surfacing technology to solve the failure of metal parts lead by  abrasion.
Composition and structure of surfacing material have an important influence on the performance of the whole component. According to the composition of hardface welding metal and the structure of surfacing layer, hardfacing alloys can be classified as iron-based, nickel-based, cobalt-based, copper-based and tungsten carbide and so on.

Iron-based alloys
Iron-based alloys are the most widely used hardfacing welding materials. It adds other alloying elements such as Cr, Mo, W, Mn, Si, V, Ni, Ti, B, etc based on the carbon which not only affects the formation of hardening phase in the surfacing layer but also affects the properties of the body structure. The biggest advantage of iron-based hardfacing materials is its lower cost. Due to the carbon content, alloy content and cooling speed, the structure of the surfacing layer can be pearlite, martensite, austenite and carbide, etc.
1.Pearlitic steel surfacing material. This alloy has good weldability, strong impact resistance but low hardness. It is mainly used for repairing mechanical parts of elephant shafts.
2. Austenitic steel surfacing material. Austenite has high impact toughness, good corrosion resistance and heat resistance. It is generally used to repair parts with intermetallic and abrasive wear under severe impact loads such as mine trucks and railway turnouts.
3. Martensitic steel surfacing material. The hardness, yield strength and wear resistance of Martensite building-up welding layer are higher and it can withstand medium impact strength, but its impact resistance is worse than that of pearlite steel and austenitic steel layer. Mainly used for repairing intermetallic wear parts, such as gears, tractor chassis, etc.
4. Alloy cast iron bead welding material. This kind of welding layer has high abrasive wear resistance, heat resistance, corrosion resistance, good oxidation resistance but mild impact resistance and easy to crack when building up welding. It is mainly used for bead welding of agricultural machinery, mining equipment and other parts.

Cobalt-based alloys
Cobalt-based bead welding metals, also known as Stellite alloys, mainly refer to cobalt-chromium-tungsten alloys, which can maintain high strength and hardness at about 650 C and have excellent corrosion resistance and wear resistance. Among these kinds of bead welding metals, cobalt-based alloys have the best comprehensive properties and are mainly used for surfacing parts at high temperature.

Nickel-based alloy 
Nickel-based alloys are the most widely used for hardfacing and it has excellent wear resistance, corrosion resistance, heat resistance and high-temperature oxidation resistance. They are usually used in corrosive media or high-temperature environments where low-stress wear occurs. Nickel-based alloys containing intermetallics such as Hastelloy C-22, are more suitable for tungsten gas shielded arc surfacing or plasma arc surfacing and used to surfacing the sealing surface of valves working in severely corrosive media. Nickel-based alloys containing carbides are cheaper than cobalt-based alloys and are ideal substitutes for cobalt-based surfacing metals.

Copper-based alloys
Copper-based bead welding metals are characterized for their excellent corrosion resistance, cavitation resistance and intermetallic wear resistance. They can be hardfaced on iron-based materials to make bimetallic parts and repairing worn parts. However, its ve poor sulfide corrosion resistance and high temperature creep resistance made it not easy to weld and are only suitable for environments below 200℃. This kind of bead welding metal is mainly used for welding of bearing bush, sealing surface of the low-pressure valve, etc.

Tungsten carbide alloys
Tungsten carbide hardfacing alloy wire is famous for its high hardness, wear resistance, strength and high forward elastic modulus. It has the highest wear resistance among all surfacing alloys and has become irreplaceable composite surfacing alloys for workpieces or surfaces under severe abrasive wear and gas-particle wear. It is widely used in oil drilling, metallurgical mining and coal mining, civil construction, building materials, sugar, power generation, agricultural machinery and other industries

The processing property of metal refers to the possibility or difficulty of obtaining qualified products in the cold or hot manufacturing process of mechanical parts, that is, the ability of materials to adapt to the practical production process requirements. Different processing conditions lead to different processing methods and product properties, such as casting, forging, deep drawing, bending, cutting, weldability, hardenability, etc. Process performance is often determined by a combination of complex factors (physical, chemical, mechanical ans so on).

 

1. Castability

The ability of metal materials to obtain qualified workpieces by casting which measured by fluidity, shrinkage and segregation. Fluidity is the ability of liquid metal to fill a mold. Shrinkage refers to the degree of volume shrinkage during solidification and segregation refers to the inhomogeneity of chemical composition and structure in metal due to the difference of crystallization sequence in the process of metal cooling and solidification.

2. Forgeability

It refers to the metal material can change the shape without crack performance in the pressure processing, that is the capacity, in the hot or cold environment, metal can be hammer forging, rolling, stretching, extrusion and other processing. Malleability is mainly related to the chemical composition of metal materials.

3. Machinability

Machinability refers to the difficulty to become qualified workpieces in the cutting process. Machinability is often measured by the surface roughness, the allowable cutting speed and the abration of tool. This is not only related to the chemical composition and mechanical properties itself, but also related to the cutting process (like tool geometry, durability, cutting speed and feed quantity, etc.). Although there are many factors affecting the cutting performance, but the most important is the nature of the metal itself, especially the hardness, when the metal hardness of HB150~230, the best cutting performance.

4. Weldability

Weldability refers to the adaptability of metal materials to welding processing, that’s the performance of obtaining qualified welded joints under specified welding conditions. It related to metal welding defect sensitivity and the performance welding joints to meet the use requirements under a certain welding process conditions.

5. Heat treatment

Metal heat treatment is to heat the metal workpiece to the appropriate temperature for a certain time and then cooling at different speeds in different media, by changing the surface or internal microstructure of metal materials to control its performance of a process. Heat treatment mainly includes annealing, normalizing, quenching, tempering, tempering, chemical heat treatment, solid solution treatment, precipitation hardening (precipitation strengthening), aging treatment and so on. The heat treatment performance of steel mainly considers its hardenability(quenching can get higher hardness and smooth surface), containing manganese, chromium, nickel and other elements of alloy steel hardenability is better while carbon steel hardenability is poor. The heat treatment requirement of aluminum alloy is strict and only several kinds of Copper alloy can be strengthened by melting heat treatment.

 

 

GH3600(ASTM/ASME UNS N06600) is a high temperature and oxidation resistant nickel based alloy. It is widely used in petrochemical processing, nuclear power and aerospace industry. Aerospace GH3600 alloy tubes are mainly used for hydrogen and oxygen rocket engines of upper stage power devices of carrier rockets. At present, the United States is planning on the basis of hydrogen and oxygen rocket engine’s orbit transfer vehicle and single stage to orbit of the space shuttle, developed for landing on the moon of Saturn rocket, second, third class, the J – 2 lox/lh2 engine developed by France and other European countries Ariane rocket and Japan is developing, the carrier rocket level above all use hydrogen engine as their main power devices.

At present, forging and drilling or cross-rolling piercing technology is mainly used in the billet of Nickel-based pipes in China. Due to the limitation of forging ratio, the uneven grain size of tube billet and a waste of raw materials in the drilling process, forging and drilling process are generally only suitable for thick wall, a short length and difficult to prepare hot-processed tube billet. Although the cross-rolling piercing process has high efficiency and high yield, due to its own structural constraints, whether it is two-roll or three-roll cross-rolling piercing, the material is not subjected to three-dimensional compressive stress, which results in circular micro-cracks on the cross-section of the billet and affects the possibility of subsequent processing. GH3600 alloy pipes for aerospace use are extruded to avoid the above situation.  Operations details including:

The equipment of extrusion blank is 2.5MN horizontal extrusion press. The blank size is 125 mm x 37 mm x L. The extrusion force is 1.6-1.8MN. The extrusion parameters are heating temperature 960-980 C, holding time 90-120 min, preheating temperature 500-600 C, extrusion speed 80-100mm?S-1 and extrusion ratio 10.1.

Two different rate distribution methods were used for cold rolling of finished products. The effects of different cold rolling processes on mechanical properties of products were compared for different cold rolling processes from 15mm *1.75mm to 5.8mm *0.4mm. The annealing pass processing rate of process 1 ranges from 68.9% to 70%, and that of process 2 ranges from 51.5% to 56.9%. The results are as follows:

• GH3600 alloy tube can be extruded to refine the microstructure obviously and improve the microstructure uniformity.
• The cold processing rate of finished products is controlled within the range of 51% to 57%, which can not only obtain good finished product structure, but also improve the room temperature elongation and yield strength at high temperature, and obtain better performance of finished products. Elongation of 39.8% at room temperature and 900 ℃ under the Rm of 124 Mpa, Rp0.2 is 82 Mpa.

The key point of tube extrusion is the matching of extrusion ratio and extrusion temperature. High extrusion ratio is beneficial to obtain the more uniform structure, but it will increase the extrusion pressure at the same time. While high extrusion temperature reduces the extrusion pressure, it also reduces the surface quality and dimensional accuracy of the product. At the same time, the sheath and lubrication mode of the high temperature alloy extrusion also play a crucial role.

 

 

Titanium is silver colored and one of the most corrosion-resistant structural metals. It took 120 years from the first titanium discovery in the late 18th century to the production of pure titanium and then the metallurgist spent nearly 40 years put the pure titanium obtained in the laboratory into the industrial production. Many researchers have done plenty of experiments. In 1948, Dupont company finally successfully produced tonnage porous and irregular shape titanium, that is, sponge titanium.

The production principle of sponge titanium is to reduce TiCl4 by Mg and generate Ti and MgCl and then vacuum distillation to remove the residual Mg and MgCl2 to obtain a pure, spongy, multi-empty titanium lump. The titanium lump is taken out, peeled, cut, and crushed to make sponge titanium.

During the reduction, distillation process, influenced by the complex factors such as temperature, pressure, furnace condition and segregation, there exist some impurities in titanium sponge lump. According to Chinese standard GB/T 2524, titanium sponge can be classified 0-5 levels, a total of six grades, namely the MHT 100, MHT 110, MHT120, MHT140, MHT160, MHT200 (The number refers to Brinell hardness maximum values), purity (%) quality generally is 99.1 ~ 99.7, total impurity elements (%) of 0.3 ~ 0.9, impurity element oxygen quality (%) of 0.06 ~ 0.30, hardness (HB) is 100 ~ 200.

 

How was the titanium sponge packaged?

Accoding to Chinese national standard: sponge titanium is packed according to the net weight of 70kg ~ 250kg per barrel (piece), the packing barrel is galvanized iron barrel lined with polyvinyl chloride film bag, sealed with open cover, the junction of the barrel cover and the barrel body should be able to identify whether the package is intact. It’s usually 200 kilograms a barrel now for titanium sponge manufacturer. After packaging, the barrel is pumped out and filled with argon and should be stored in a dry warehouse, not piled up in the open air or mixed with acid, alkali and other corrosive items. The sponge titanium should be handled with care during transportation to prevent dampness and damage of seal.

 

How to maintain and storage titanium sponge?

Titanium sponge oxidizes easily. Its large exposed area can absorb a lot of oxygen, the oxidized sponge titanium is impossible to remove oxygen elements in the casting of titanium ingots unlike steel making and only be discarded.

Long-term storage of titanium sponge is costly and dangerous. It’s said that sponge titanium can be stored for three years with argon gas filled in PVC film bags, but it was denied by titanium processing plant. Generally speaking, the longest period of one year is appropriate. Besides, the storage of sponge titanium must require regular rotation, generally one-year rotation, so that the storage cost of sponge titanium will be very high.

Spongy titanium Storage is dangerous. The titanium sponge allows 5% powder, so that in the processing and treatment of titanium sponge if dust clouds produced, it is easy to cause combustion when met open fire or electrostatic fire.

In conclusion, sponge titanium is not suitable for long-term storage. The porous “sponge titanium” cannot be used directly and must be melted into liquid in an electric furnace before it can be cast into ingots. Titanium ingots are in a dense state, which is a necessary process in the titanium production and is convenient for loading, unloading and handling. Therefore, in terms of the advantages like the low cost of maintenance, fire and shock resistant, recyclable, economy and convenience, it is quite appropriate to store titanium ingots.

 

Sponge titanium production is the basis of titanium industry and titanium sponge is an intermediate product in its purest form and material for titanium alloy. It is mainly used to produce titanium ingot, which in turn is used to make slab, billet, pipe, bar, plate, sheet, and other titanium mill products such as titanium alloys, billets, ingots etc.