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Hot-working AISI H13 tool steel offers high hardenability, excellent wear resistance and hot toughness, has been widely used in hot forging dies, pressure dies casting tools, extrusion tools, hot shear blades, stamping dies, plastic mold and aluminum alloy die casting dies, is the most commonly used hot-worked die steel. The H13 steel made by electroslag remelting (ESR) process can effectively improve the low microstructure and densification of steel, and improve the isotropy of die steel. Compared with ESR process, the furnace refining H13 can save 20% ~ 30% of the production cost, is still the mainstream smelting method. Reasonable forging process and heat treatment process can improve the quality, performance and service life of H13 steel.

The heat treatment temperature and cooling method depend on the critical transition point and isothermal transition of the H13 tool steel. The following data you should know before the heat treatment of H13 steel:

1) Critical point: Ac1, 850~885 ℃, Ac3:910 ℃.

2) Cooling transition point: Ar1, 700℃; Ar3, 820 ℃; Ms, 335 ℃.

3) Austenitization temperature: 1 010 ℃

Annealing

In order to eliminate the stress of H13 steel forging, improve the structure, refine the grain, reduce the hardness for machining, annealing is a necessary process, generally is performed high temperature/isothermal spheroidization annealing: 860 ~ 890℃, heating and holding for 2h, cooling to 740 ~ 760℃ isothermal 4h, furnace cold to about 500℃ out of the furnace.

(1) the complete annealing process of H13 steel is :850~900℃, 3~4h.

(2) the isothermal spheroidizing annealing process: 845 ~ 900 ℃ by 2 ~ 4 h/furnace cooling + 700 ~ 740 ℃ by 3 ~ 4 h/furnace cooling, [40 ℃ / h, 500 ℃ from air cooling];

(3) H13 steel dies with higher quality requirements shall also be annealed to prevent white spot and the process cycle shall be longer;

(4) for molds with complex shapes, a stress-free annealing shall be conducted after rough machining :600~650℃, 2h/ furnace cooling; The carbide structure of large H13 steel forgings treated by conventional spheroidization annealing is extremely uneven, and the existence of severe intergranular carbide chains can be realized by multiple spheroidization annealing or austenitizing fast cooling (normalizing) respheroidization annealing

Quenching

H13 steel has good hardenability, for H13 forging thickness less than 150 mm, oil quenching can achieve uniform hardness, but it’s  easy to cause oxidation and decarburization and other defects deo to Mn, Si elements in the steel. It is recommended to use salt bath, controlled atmosphere heat treatment, vacuum heat treatment or coating to prevent decarburization.

The hardness of 54~55 HRC can be obtained by quenching at 1 030 ℃, and the grains begin to grow beyond 1 040 ℃. Therefore, the heat treatment temperature range of 1 030~1 040 ℃ is recommended. At the same time, special attention should be paid to pre-cooling to 20~30 ℃(950~980 ℃) above Ac3 when coming out of the oven to reduce stress concentration and avoid cracking.

Heating temperature 1020 ~ 1050℃, oil cold or air cold, hardness 54 ~ 58HRC;It is required that the quenching process specification of the die mainly hot and hard, the heating temperature is 1050 ~ 1080℃, the oil is cold, and the hardness is 56 ~ 58HRC.

Tempering

In order to eliminate the stress and improve the high temp toughness of H13 forgings must be tempered at high temperatures, secondary tempering can be used to improve the life of the die due to the good fire resistance and secondary hardening of the alloy elements in the steel. The tempering temperature (580±20 ℃) was used to obtain the hardness of 47~52 HRC. The microstructure after tempering is tempered martensite and a small amount of granular carbides.

Tempering should be done twice. When tempering at 500℃, the secondary hardening peak appears, with the highest tempering hardness and peak value around 55HRC, but the worst toughness. Therefore, according to the use of the mold needs 540 ~ 620℃ tempering is better. Quenching heating shall be preheated twice (600 ~ 650℃, 800 ~ 850℃) to reduce the thermal stress generated during heating.

Titanium and its alloy have low density and excellent strength and weight ratio, good corrosion resistance and high mechanical strength, but expensive production cost. Titanium and titanium alloy grinding and polishing’s low efficiency make its microscopic structure change because the excessive severe cutting and polishing process will create a mechanical twin in the alpha phase.

At present, the methods of free abrasive grinding and chemical mechanical polishing are mainly used in the precision machining of titanium alloys. Grinding fluid is made by free silicon carbide or alumina abrasives, polishing fluid is a mainly strong acid, strong alkali or toxic chemical reagents, the free abrasives are difficult to control its movement trajectory, easy to leave deep scratches on the surface of the processing, reducing the processing accuracy. The handling of strong acids, strong bases and toxic chemicals is cumbersome, time-consuming and potentially hazardous to operators and the environment.

Early mechanical polishing processes were time-consuming, and almost all mechanical polishing methods used a polishing fluid containing an erosive agent in the final or two-step process. The electrolytic polishing methods can often get a better polishing surface, but the electrolyte brings a certain risk. This article here discusses the methods of grinding and chemical mechanical polishing to achieve super precision polishing of titanium alloys.

In the 1970s and 1980s, engineers Springer and Ahmed first published a paper on the polishing method of titanium and titanium alloys in 1984. This is the three-step sample polishing method. It is assumed that 320 grit paper is used to finish the sample grinding process, but this may not always be the case. If the sample is cut with an ultra-thin cutting piece or a grinding wheel cutting piece with appropriate bonding strength, the cutting surface is smooth and the damaged layer is minimal, 320grit is the ideal choice. If the cut surface is rough and the damaged layer is large, for example, if a band saw is used, then rougher sandpaper must be used and a certain amount of time must be spent to remove the damaged layer.

Springer, Ahmed titanium 3-steps polishing methods

1. Grind, water cool with 320grit paper, grind for 2-3 minutes, remove the damage layer caused by cutting and make the surface of the sample flat. 320grit SiC sandpaper, water-cooled, rotate at 240 RPM, turn in the same direction, pressure: 27N (6lbs)/each sample until the sample is smooth. Note: removal of cut damage layer is the foundation of polishing, incomplete removal can directly affect the experimental results.
2. Rough polishing, pre-apply 9 mm METADI® diamond polishing paste on TEXMET® polishing cloth with holes, use distilled water as cooling lubricant, and polish for 10~15 minutes. Rough polishing process: 9 mm METADI diamond polishing fluid + METADI polishing lubricant, polishing surface with ultra-pad ™, 120 RPM, reverse rotation, pressure: 27N (6lbs)/each sample, time: 10min.
3. Finish polishing, using MICROCLOTH® or MASTERTEX® polishing cloth, adding MASTERMET® silica suspension polishing fluid and polishing for 10-15 minutes. Final polishing process: on the polished surface of MICROCLOTH, use MASTERMET silica polishing fluid, rotate at 120 RPM, reverse rotate, pressure: 27N (6lbs)/sample, time: 10min.

Müller titanium 3-steps polishing methods

1. P500 sandpaper, water-cooled, rotating speed 300 RPM, pressure 16.7n (3.75lb) on each sample, preparation time until all samples are smooth.
2. P1200 sandpaper was water-cooled at a speed of 300 RPM, a pressure of 16.7n (3.75lb) on each sample, and a 30S preparation time. Note: the specific time is determined according to the actual polishing situation, and the time parameters are only for reference. Usually, manual polishing is used for polishing, so the parameters may vary depending on the equipment.
3. Synthetic non-pile polishing cloth + silica suspension polishing fluid containing chemical etchant, polishing machine speed is 150 RPM, polishing time: pressure on each sample is 33N (7.5lb) for 10 minutes, pressure on each sample is 16.7n (3.75lb) for 2 minutes, and pressure on each sample is 8N (2lb) for 1 minute.
4. Polishing agent: 260ml SiO2+40ml H2O2 (concentration 30%),1mlHNO3 + 0.5ml HF.The P500 and P1200 grit sizes of FEPA correspond to ANSI/CAMI 320/360 and 600 grit, respectively.

AISI S7 steel was originally derived from the United States commercial brand Finkl DRX, is a kind of medium carbon chromium molybdenum alloy shock resisting tool steel. It’s characterized by a good comprehensive property like hardenability, high strength, toughness, tempering stability and oxidation resistance of temperature, is suitable for load tools or dies working under high temperature and high impact and hammer forging die manufacturing. This steel has a high hardness after forging, so it needs softening and annealing to reduce the hardness and facilitate cutting. The available forms of S7 steel including round bar, drill rod, rectangular bar, ground flat bar, square bar and plate

Equivalent Material

S7 Chemical Composition

S7 Mechanical Property

Comparison with  A2 and H13

According to the analysis of hardness, wear-resistance and toughness (see the figure below), the performance of S7 is between A2 and H13, which can make up for the deficiency of both.

Hardness

The applied hardness of S7 is between 52~58HRC. Another 1~2HRC can be raised if proper heat treatment is applied.

Applications

S7 has high strength, toughness and wear resistance， making it a tool steel used for pneumatic tools and cutting tools such as punch, forging die, hammer, chisel, jig die, cutting tool and various cold and hot dies that require high hardness.

For more information about the S7 tool steel, contact us now!

Vanadium(V) has been widely used in metallurgy, chemistry, aerospace industry, agriculture, medicine and other fields due to its characteristics of vanadium added in steel can improve the strength, toughness and plasticity of steel, improve the hardness,  abrasion resistance in steel products, among which the metallurgical industry accounts for almost one third of its consumption. At present, V has been added in high-strength hot-rolled ribbed steel, high-carbon low-alloy steel, spring steel, bearing steel, mold steel, high-speed steel, Martensite heat-resistant steel and other steel as an important alloying element to improve the performance of steel. In the last article, we introduce the effect of V in steel, here we will continue the use of vanadium in steel, if interested, please read on!

For high carbon and low alloy, V is mainly used to refine grain, improve the strength, yield ratio and low-temperature toughness after normalizing, and improve the welding performance of steel. V will reduce the hardenability of steel in general heat treatment conditions, usually added with one or two kinds of alloy elements like manganese, chromium, molybdenum and tungsten. High strength low alloy steel contains 0.04% ~ 0.12% V, and special steel can up to 0.16% ~ 0.25%.

Vanadium is an indispensable alloy element in high-speed tool steel(HSS). It can prevent grain growth, improve red hardness and cutting ability of steel, increase wear resistance and prolong service life in high-speed tool steel containing tungsten. Almost all-alloy mold steel like cold work tool steel, hot work tool steel, plastic mold steel contains 1% ~ 3% vanadium, a few special requirements can be up to 5%. Vanadium is the main secondary hardening element that is commonly used hot mold steel (H13) and cold mold steel (D2). The content of V in mold steel is usually 0.1% ~ 5%, the United States developed A11 cold tool steel of which vanadium content up to 9.75%. In Germany, V consumption in tool steel and high-speed steel accounts for about 1/3 of the total V consumption. These steel products made by HSS can reach to 60HRC hardness, has been widely used surgical instruments and tools and various cutting tools: drills, taps, milling cutters, tool bits, gear cutters, saw blades, planer and jointer blades, router bits; sharp tools like files, chisels, hand plane blades, knives

In addition, V has a place in heat resistant steel, stainless steel, bearing steel and spring steel, Nickel-based alloys. Vanadium addition of 0.15% ~ 0.40% can form highly dispersed carbide and nitrite particles in heat-resistant steel, which polymerizes and grows very slowly at a high temperature, which can improve the thermal strength and creep resistance of heat-resistant steel and be applied in power station systems such as T91 and P92 steel.

For spring steel and bearing steel, V can improve the elastic limit, strength and yield ratio, reduce the decarburization sensitivity of the steel during heat treatment, thereby improving the metallurgical and surface quality of the steel. Vanadium has also been used in Hastelloy corrosion-resistant alloy, for example, Hastelloy B alloy contains V≤0.60%, Hastelloy C22, Hastelloy C 276 alloy ≤0.35%, Hastelloy N ≤0.50% V, Hastelloy W ≤0.60% V.

H13 is the most commonly used hot work steel, it has higher thermal strength and hardness, wear resistance and toughness, better heat resistance fatigue performance, has been widely used in the manufacture of various forging die, hot extrusion die and aluminum, copper and its alloy casting mold. Hot-working tool steel undertakes a lot of impact load, friction, intense cold and heat cycle caused by thermal stress and high-temperature oxidation, often produce a series of failure forms such as crack, collapse, wear and so on.

H13 steel is a type of hypereutectoid alloy steel, and its metallographic structure has many defects such as non-metallic inclusions, carbide segregation, loose center and white spots, which can reduce the strength, toughness and thermal fatigue resistance of die steel. Heat treatment technology has a great influence on the structure and performance of the H13 steel mold.

Forging process
H13 steel contains quite high alloy elements, offering poor thermal conductivity and low eutectic temperature, easy to cause overburning. For the blank with Ø 70 mm H13 steel block in diameter, it should be preheated within the range of 800 ~ 900 ℃, at first, then in the forging heating temperature 1065 ~ 1175 ℃, repeatedly pulling long upsetting forging, be noted that the forging ratio should be greater than 4.

Spheroidization annealing process
The purpose of spheroidization annealing is to uniform structure, reduce hardness, improve cutting performance and prepare the structure for quenching and tempering. The annealing process is insulated at 845 ~ 900℃ (1h+1min) /mm, then cooled to 720 ~ 740℃ isothermal (2h+1min) /mm, and finally cooled to 500℃ and out of the furnace, spheroidization annealing structure is pellet pearlite and hardness less than 229HBS.

Quenching and tempering process
The best heat treatment process of H13 steel is that the oil cold quenched or fractional quenched after heating at 1020 ~ 1080℃, and then it is tempered at 560 ~ 600℃ twice. The microstructure is tempered torstenite + tempered sostenite + residual carbide, and the microhardness is 48 ~ 52HRC. For high thermal hardness requirements of the mold (die casting die) can be taken upper limit heating temperature quenching. The lower limit heating temperature can be used to quench the mold (hot forging mold).

In addition, the manufacture of H13 steel mold has to go through a series of processes such as forging, annealing and machining. Improper operation in each process will cause premature failure of the mold and reduce its service life. Therefore, attention must be paid to the influence of preheating, cooling and lubrication of the mold, forging, cutting, grinding and EDM on H13.