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Hot work tool steel is the high alloyed Cr-Mo-V tool steel which used for the non-cutting forming of workpieces made of iron and alloys at high temperatures. They are applied in processing such as hot forging dies, extrusion, drop forging as well as tube and glass products manufacturing. In recent years, in order to meet the large and complex shape of hot processed workpieces, there produce higher requirements for the mold load and tool steel performance. Therefore, many countries and tool steel mills continue to develop standards for mold steel and tool steel themselves.

Due to the different environment(application, temperature, pressure, atmosphere) of hot working tool steel, the mold bears a variety of pressure. In order to reduce the melting loss of casting dies, it is not enough only to improve the performance of tool steel, but also the surface treatment technology. In order to improve the service life of die, tool steel should be selected according to the service environment, tech design and other aspects such as:

1.High-temperature strength

The hot forging die and casting die contact directly with the processed material at high temperature, making the surface of the die soften and the surface strength of the die decrease. In addition, under the action of forming pressure and thermal stress, the surface of the mold is prone to crack and bond, which affects the appearance of processed workpieces. Therefore, the requirement of tool steel is to restrain high temperature softening and keep the strength (softening resistance) of steel. The secondary carbides in hot working tool steels after quenching and tempering play an important role in improving the softening resistance of steels. The addition of Mo, W, V and other alloying elements has a good effect on improving the softening resistance. However, excessive alloying elements will lead to component segregation and the increase of carbides, resulting in a decrease in toughness. Thus hot work tool steel must have both softening resistance and toughness.

2.The toughness

Mold cracks occur in the corner of the mold, machining defects and hot cracks and other stress concentration parts. But the toughness of steel will affect the crack resistance of die steel. Charpy impact value and fracture toughness value are the most commonly used toughness evaluation indexes. Cr, Mo, W and V carbides in hot working tool steel play a great role in improving the softening resistance and hardening hardness. However, the excess carbides are the main reason for the decrease of toughness of tool steels. Therefore, according to the need of strength – toughness balance, the composition of hot work tool steel should be designed reasonably. Ni is an element that does not form carbides, but the solid solution in tool steel matrix can improve its toughness.

The toughness of steel is closely related to the cooling speed of steel. During the quenching process of large molds, the cooling speed of the center is lower than the cooling speed of the surface, the normal and uniform quenching structure in the center is often not obtained, resulting in a decrease in toughness. Therefore, the hardenability of steel should be improved. Cr, Mn, Mo and V are effective elements to improve the hardenability. The evaluation methods of quenchability include continuous cooling transition curve (CCT) and Charpy impact value of slow cooling quenching in semi-cooling time. The semi-cooling time is the time when the temperature of steel decreases from quenching temperature to 1/2 quenching temperature. The operation of setting various semi-cooling time can simulate the cooling speed of the large mold center.

3.The thermal cracking resistance

Hot working tool steel mold, especially casting mold which requires workpiece with a flawless appearance, often occurs hot – cracking. These tortoiseshell cracks in the mold use process expansion, growth, resulting in cracking. The causes of hot cracks are that the surface of the mold is softened by the heat of the processed workpiece, the internal oxidation of the existing cracks caused by the contact between the mold and the atmosphere, and the local expansion and contraction caused by repeated heating and cooling heat cycle promote the crack expansion. In order to restrain the occurrence of hot cracks, the high temperature strength (softening resistance) and toughness of tool steel should be improved.

4.Melting resistance

Melting loss is a special damage form of casting die. The aluminum and magnesium ejected react with the mold material to alloying the surface of the mold, resulting in mold wear and thermal bonding. This phenomenon is called melting loss. The effective method to prevent melting loss is to avoid contact between liquid aluminum and liquid magnesium and Fe, the main component of mold material. Diffusion treatment including sulfur nitriding and coating treatment (PVD, CVD) surface treatment are the most effective methods.

 

Lkalloy Steel is a leading exporter and supplier of H13 hot work steel in plates, blocks and flat /square /round bars, we provide AISI H13 tool and die steel in all sizes as your requirements. Consult our team for h13 steel projects.

 

In the last article, we discussed “what’s the admiralty brass”, today let us take the “brass” on the table. As the two of frequently used of copper alloy, they indeed have various of similarities more than the literalness, sometimes brass and bronze are a source of confusion and people often mix them up, Can you tell the difference between them? If you are facing the same confusion, this article will be a timely help.

Alloying difference

Both brass and bronze include a small range of other elements including arsenic, lead, phosphorus, aluminum, manganese, and silicon. Brass is an alloy of copper and zinc, in proportions which can be varied to achieve varying mechanical and electrical properties. In contrast, bronze is an alloy of copper and tin. There are many different bronze alloys, but typically modern bronze is 88% copper and 12% tin.

Surface difference

The most intuitive and visual difference is the color of the metal surface. Bronze is characterized by its reddish brown color (depending on the bronze alloys) while brass has a dull-gold surface. Brass can range in color from red to yellow depending on the amount of zinc added to the alloy. You can also tell the difference between brass and bronze from its surface.

Property difference

Brass has higher malleability than bronze. The relatively low melting point of brass various from 900 to 940℃, depending on composition. Its flow characteristics make it a relatively easy material to cast. By varying the proportions of copper and zinc, the properties of the brass can be changed, allowing hard and soft brasses.

Compared with brass, bronze is hard and brittle. It has a higher melting point and melts at 950℃, that also depends on the amount of tin percent. Bronze resists seawater corrosion and metal fatigue more than steel and is also a better conductor of heat and electricity than most steels.

Application difference

The different property leads to the difference of application. Brass is commonly used in the production of some complex shaped stamping products, such as condensing pipe, drainage pipe, shell and so on, or some small hardware, mechanical parts. Brass is also used for decoration for its bright gold-like appearance or where need low friction such as locks, gears, bearings, doorknobs, musical instruments and even zippers.

Because of its good performance of resistance to corrosion and hardness, Bronze is used in some elastic elements, cables or conductive materials even sculptures, musical instruments and medals, and in industrial and nautical applications such as bushings and bearings.

Price difference

The price varies depending on which grades you are comparing and alloying elements mixed in. There are over 40 standard grades of brass with a zinc content varying from around 36-42%, while bronze with about 6- 12% tin. The price of zinc is lower than copper while the tin is high than copper, sometimes due to the manufacturing processes, for casting or forging. In general, bronze is usually more expensive than brass.

While these two “red metals” may look similar, they are actually quite different. Visit our other copper metals, click here

 

References:

http://en.wikipedia.org/wiki/Brass

 

 

Titanium offers an amazing corrosion resistance but is two times lighter than steel. Titanium also has a very high tendency to oxidize at higher temperatures. It extremely important to keep the molten metal away from atmospheric air while welding because even very little contamination of oxygen will lead to porosity. With the wide application of titanium and titanium alloy, the progress of welding technology gives us more choices. It not only saves the material, but also reduces the quality of the whole workpiece. Here we will introduce the 4 kinds of commonly used welding methods: Tungsten Inert Gas arc welding, metal inert gas welding, laser braze welding, vacuum electron beam welding and so on.

 

Tungsten Inert Gas arc welding（TIG）

 

TIG is the best-welded process for titanium alloy plate and tube with a thickness below 3mm. Tig welding can be divided into open welding and pool welding or manual welding and automatic welding according to the methods.

gas tungsten arc welding in the atmospheric environment is using the shielding and purge gas to welding nozzle, drag cover and backside protective device to separate the welding high-temperature area from the air, so as to prevent the air from invading and contaminating the metal in the welding area. This is a type of local gas protection welding method. When the welded parts are complex in structure and difficult to finish the protective cover or back side, the welding inside the weld pool should be adopted. The pool body should be vacuumed before welding and then filled with argon or argon helium mixture. then welding inside the pool conducted under an inert atmosphere, which is a welding method of overall gas protection. Often Titanium is welded in a gas chamber with pure argon gas to make sure that the weld pool gets proper protection.

 

Metal inert-gas welding

MIG works by using a continuously feeding of welding wire that burns, melts and fuses both the base and parent metals together. You can weld a variety of materials such as mild steel, stainless steel and titanium, obviously. In this process for welding titanium and titanium alloy, the welding material and its thickness needs to be selected strictly. Generally, thin titanium plate adopts the technology of droplet transition welding, while thick plate adopts the droplet spray transition method. The effect of melting argon arc welding is excellent compared with other welding methods, mainly used for welding thick titanium alloy plate. Protective gas content and pre-welding cleaning are key factors to MIG processing.

 

Laser braze welding

Laser braze welding has incomparable advantages to other welding methods, it’s stability and automation, not affected by the magnetic field, especially suitable for precise titanium and titanium alloy pipe. As a non-uniform body, The structure and properties of welding joint are greatly changed, and the plastic damage behavior of joint is quite different from that of homogeneous material. The results show that the fatigue life of titanium alloy thin plate laser welding and active laser welding head is lower than that of base metal. The defocusing of the laser beam is also the key factor to affect the weld-forming quality.

 

Electron beam welding

From the early 1960s, as an advanced high-energy beam processing method, electron beam welding began to be applied to the welding of precious metals in the atomic energy industry, aircraft manufacturing and aerospace industry. Titanium absorbs O2 and N2 at high temperature rapidly, making the welding seam brittle, while vacuum electron beam can obtain high-quality welding joint, which is a unique advantage of vacuum electron beam welding. With the rapid development of the application of cutting-edge technologies such as aerospace, the uniqueness of materials used for aerospace parts and the particularity of welding requirements make electron beam welding quickly become a necessary process for the processing of these important parts, which is widely used in the welding of aircraft important bearing parts and engine rotor parts.

The electron beam welding of titanium alloy plate can be realized by adopting reasonable welding process. Obtaining a reliable joint is the key for electron beam when welding titanium alloy, because the reliability of joint performance will directly affect the safety of titanium alloy structure. Saresh of the National Institute of Technology Calicut in India performed single-channel electron beam welding on the 17.5mm thick titanium alloy plate, but the weld head failed to realize full penetration welding. When double-channel double-side electron beam welding is adopted, the total penetration welding of titanium alloy plate with a thickness of 17.5mm can be achieved, but the backside welding shall be followed as far as possible after the front welding is completed to avoid the fusion zone hole caused by welding joint pollution.

 

Even the mature welding technical, there are many basic factors make titanium difficult welding from other metals: a higher melting point and layers of shielding to prevent oxidation. In spite of the precautions that need to be taken, many engineers are more routinely and economically welding titanium.

 

 

Composed by close to 1:1 Ferrite and Austenite phase, duplex stainless steel has excellent mechanical properties and resistance to chloride stress corrosion performance, was widely used in the oil and gas pipelines, chemical transportation tank, and the shipbuilding industry. Duplex stainless steel has excellent weldability and during the welding process, the microstructure of welding joint, especially the thermal zone, will cause a series of complicated phase transformation. Different welding methods and process also affect the microstructure and duplex in the welding joints, resulting in changes in mechanical properties and corrosion resistance. As a nickel saving stainless steel, duplex stainless steel 2205 can take the place of Austenitic stainless steel in many cases and reduce the engineering cost effectively. It is necessary to study the effect of different welding processes on the microstructure and properties for 2205 stainless steel joints.

Recently, engineer studied the effect of heat input on Austenite shape and volume fraction in microstructure of simulated heat affected zone of duplex stainless steel and found that increasing heat input can effectively improve the volume fraction of Austenite in simulated heat affected zone. In addition, similar results can be obtained by changing the cooling rate of welded joints. Our researchers used metal inert-gas welding (MIG) to perform welding experiments on 160*320*12mm 2205 duplex stainless steel plate under different thermal input conditions to study the effect of thermal input of MIG welding on the microstructure and mechanical properties of welded joints. The results showed that：

• The microstructure of the heat zone is greatly influenced by the heat cycle of welding. In the incomplete recrystallization zone far from the fusion line, the edge undulation of the belt Austenite increases gradually with the increase of heat input, and the width of zonal Austenite increases gradually. In the rough crystal region near the fusion line, the grain boundary Austenite forms a closed structure to enclose the Ferrite which produces few Austenite structure.
• The Austenite morphology of weld metal in different areas is significantly different. The Austenite structure near the welding center is mostly equiaxial massive Austenite, while the Austenite structure near the fusion line is relatively thick and mainly weinstein-austenite. With the increase of heat input, the weinstein austenite in weld metal gradually decreased, while the number of massive austenite gradually increased.
• With the increase of thermal input, the tensile strength and yield strength of the welded joint decreased slightly. Increasing the volume fraction of austenite in the heat affected zone and weld metal, decreasing of austenite and increasing of block austenite can slightly increase the fracture elongation of the welded joint.
• The microhardness of welding joint increase and then reduce in turn from base metal to weld metal. The change of microhardness is related to the austenite volume fraction in each region of the welding joint. With the increase of thermal input, the austenite volume fraction in each region gradually increases while the microhardness decreases accordingly.

 

Titanium and titanium alloy has a perfect strength and weight ratio, good toughness and corrosion resistance. Titanium alloy is mainly used for manufacturing aircraft engine compressor parts and structural parts of missile and high-speed aircraft. In the mid-1960s, titanium and its alloys were used in the general industry to make electrodes for the electrolytic industry, condensers for power stations, heaters for oil refining and desalination and environmental pollution control devices, as well as hydrogen storage materials and shape memory alloys.

At present, the annual production capacity of titanium alloy in the world has reached more than 40,000 tons, with nearly 30 kinds of titanium alloy grades. In heat processing, impurities such as hydrogen, oxygen, nitrogen and carbon are easily absorbed. Due to the poor processability, it is difficult and complex to cut and reprocess for Ti and its alloy. Annealing is implemented to eliminate internal stress, improve plasticity and produce an optimum combination of ductility, machinability, and dimensional and structural stability. The commonly used heat treatment methods of titanium alloy including full annealing, solution and aging treatment. In addition, double annealing, isothermal annealing, dehydrogenation treatment, deformation heat treatment and other metal heat treatment processes are adopted.

 

1 Full annealing

The annealing of titanium and titanium alloys serves primarily to increase fracture toughness, ductility at room temperature, dimensional and thermal stability, and creep resistance. Generally, the β and α+β alloy is fully annealed and used as the final heat treatment.

1. The full annealing of α(alpha) titanium alloy is mainly recrystallization. The annealing temperature is usually selected in the alpha phase zone and in the (α+β)/β phase transus temperature between 120 ~ 200 ℃. If the temperature is too low, it will cause the incomplete recrystallization or oxidation and coarse grains if it’s too high.
2. The full annealing temperature of the nearly dilated titanium alloy and the bestial titanium alloy was selected from the point of view of the first phase, and below the point of view of the second phase change of the first phase. In the annealing process, there are not only the tissue recrystallization, but also the changes in composition, quantity and shape of the α and β phases.
3. β (Beta) Annealing. Beta annealing is done at temperatures above the β transus of the alloy being annealed. To prevent excessive grain growth, the temperature for β annealing should be only slightly higher than the β transus. Annealing times are dependent on section thickness and should be sufficient for complete transformation. Due to the fact that the maltitanium alloy can be strengthened by heat treatment and its strength is improved after annealing, it is actually a kind of solid solution treatment.

 

2 Stress Relief Annealing

In order to eliminate the internal stress generated during pressure processing, mechanical processing and welding, to prevent chemical erosion and reduce deformation in some corrosive environments. Titanium alloy should be stress relief annealing. The stress annealing temperature is less than the recrystallization temperature, generally for 450 ~ 650 ℃. The Solution time is respectively 0.25~4h for industrial pure titanium, 0.5 ~ 2h for mechanical parts, and 2 ~ 12h for welding parts, which are then cooled in the air.

 

3 Solution Treatment and Aging

In order to improve its strength, α and stable β phase titanium alloy cannot be subjected to intensive heat treatment. The solid solution and aging treatment are to rapidly cool down from the high-temperature area to produce a higher ratio of β phase. This partitioning of phases is maintained by quenching; on subsequent aging, decomposition of the unstable β phase occurs, providing high strength so as to strengthen the alloy.

 

4 Double annealing

Double annealing improves the plasticity and toughness of the two-phase alloy and stabilizes the microstructure. The first anneal temperature is higher than or close to the recrystallization temperature, so that the recrystallization process is fully carried out and then air cooled. Because the tissue is not stable enough after annealing, a second anneal is needed, which is then heated to a lower temperature before and kept for a long time, so that the optical phase is fully decomposed and aggregated to guarantee the stability of the tissue. double annealing can also be used for Gr5 titanium alloy.

 

5 Isothermal Annealing

It’s appropriate for α+β Ti alloy. Due to its high content of stable β phase, it is difficult to fully decompose when air cooled to obtain satisfactory softening effect with full annealing. Therefore, isothermal annealing is often adopted. Heated the titanium alloy to (alpha + beta)/beta phase transition point below 30 ~ 80 ℃ and then furnace cooling, or remove the artifact to lower than the transformation temperature of 300 ~ 400 ℃ isothermal a period of time, and then air cooling. Isothermal annealing can improve the titanium plate’s plasticity and thermal stability.

 

6 Dehydrogenation process

Dehydrogenation process aims to eliminate hydrogen embrittlement. Dehydrogenation is carried out in a vacuum furnace, where heat causes hydrogen to escape from the Ti alloy, also known as vacuum annealing. The annealing temperature is 540 ~ 760 ℃, holding time 2 ~ 4 h after air cooling, vacuum degree of vacuum furnace values are not greater than 1.33 Pa. Time and temperature combinations for solution treating are given in Table below.

 

Grades	Heat treatment	Temperature /℃	Solution time	Cooling code
CP titanium	Full Annealing	630-815(sheet/plate/pipe)	0.25-2h	air cooling or slowly air cooling
		630-815(bar/wire/forging)	1-2h
Ti-5AL-2.5Sn/gr6	Full Annealing	700-850(plate)	10min-2h	air cooling
		700-850(bar/forging)	1-4h	air cooling/water cooling
Ti-0.2Pd/gr7	Full Annealing	650-760	6min-2h	air cooling/furnace cooling
Ti-0.3Mo-0.8Ni/gr12	Full Annealing	650-760	0.25-4h	Air cooling/ stepped cooling
Ti-6Al-4V	Full annealing	700-850(plate)	0.5-2h	Air cooling
		700-850(bar/forging)	1-2h	Air cooling/water cooling
Ti-6.5AL-3.5Mo-1.5Zr-0.3Si/bt9	double annealing	950	1-2h	Air cooling to 530℃
		530	6h	Air cooling