P20 Steel Grade Names | 1.2311, P20, Regional Market Terms

Common P20 steel grades include 1.2311 (DIN) and P20 (AISI). In China, it is commonly referred to as pre-hardened plastic mold steel.

Factory hardness is generally HRC 28–32, with a carbon content of about 0.35%–0.45%.

When selecting the material, its chemical composition and hardness range should be verified to ensure it meets mold polishing and machining requirements.

1.2311

Why is it called 1.2311?

On May 18, 1917, a standards association was established in Berlin, Germany. At the time, metallurgical laboratories were filled with hundreds or even thousands of metal formulas, and technicians urgently needed a numerical system that would make materials easier to identify and retrieve. Tool steels, stainless steels, and structural steels produced in steel furnaces were all assigned five-digit serial numbers.

This all-numeric system replaced the old practice of each steel mill inventing its own naming rules. In German industrial standards manuals, the classification of metallic materials appears right at the front.

· Codes beginning with 0 were assigned to pig iron and ferroalloys

· Codes beginning with 1 were assigned to all steels

· Codes beginning with 2 were assigned to heavy metals such as copper and zinc

· Codes beginning with 3 were reserved for light metals such as aluminum and magnesium

When steelworkers poured scrap steel and molten iron into an open-hearth furnace to make alloy tool steel, the first digit had to be 1. A materials testing center in Düsseldorf was responsible for further subclassifying all steels beginning with “1.”

In this five-digit system, the second and third digits together identified the specific steel group. When buyers saw “23” on a drawing, they knew the material belonged on the alloy tool steel shelf. If the molten steel from the blast furnace contained more than 0.2% carbon, workers would add chromium, molybdenum, and vanadium in strictly controlled proportions.

Steel plants in the Ruhr industrial region submitted newly formulated materials to the testing center. Review personnel heated the steels in furnaces at 800–900°C and then carried out tests such as:

· measuring hardening depth with calipers

· checking hardness loss after tempering at 200°C

· using lab spectrometers to determine the amount of retained austenite

· recording peak tensile load on a tensile testing machine

Once a formula passed testing, it received its final two digits in sequence. “11” was essentially the number of a specific file folder in the archive cabinet. By the 1950s, as more factories began producing plastic basins and television housings, workshops were rushing to buy pre-hardened steel plate with good machinability.

One steel mill submitted a new formula containing 1.5% manganese and 2.0% chromium. On the lab report, 0.40% carbon became the key threshold separating it from similar materials. The file clerk opened the classification register and stamped “11” onto the “1.23” page.

Across the ocean, the American automotive engineering system created another standard and called steel with a similar chemical makeup P20. On cargo ships crossing the Pacific, the terms P20 and 1.2311 appeared interchangeably on engineering drawings. When German factories received the material, inspectors used ultrasonic flaw detectors to scan the steel internally and look for bars stamped 1.2311.

The moving waveforms on the flaw detector screen showed the size of the impurities inside the steel. If 0.05%–0.10% sulfur was added during smelting, the last two digits changed to 12. Manganese sulfide particles inside the steel acted almost like a lubricant for milling cutters, allowing chips to break more easily and increasing machining speed by around 25%.

But sulfur reduced surface density. When molten plastic at 260°C was injected into a mold cavity, the surface of 1.2312, which contained manganese sulfide particles, could not withstand polishing with 3000-grit abrasive. The resulting plastic TV frames were covered with tiny pits. To maintain a smooth surface, sulfur in 1.2311 was kept strictly below 0.03%.

With such low sulfur content, the polished mold surface became flat and reflective. A fitter coated a wool felt pad with 3-micron diamond paste and worked it repeatedly across the mold surface. After 40 continuous hours of polishing, the surface roughness dropped to Ra 0.15 μm. Transparent plastic was then injected at 120 MPa, and the molded part reached a light transmittance of 92% after demolding.

Sometimes the procurement list also shows 1.2738. This is based on 1.2311 but with an additional 0.90%–1.20% nickel added during steelmaking. With nickel, even when a steel block thicker than 400 mm cools in air, hardness can still be maintained at the core.

When machining molds for oversized television rear covers, the total weight often exceeds 15 tons. Even when a machine tool reaches 500 mm deep into the mold, 1.2738 can still maintain 29 HRC at that depth. Without nickel, 1.2311 cools too slowly inside sections thicker than 400 mm, and the center hardness may fall to only 24 HRC.

Three advantages

Heavy trucks carry 1.2311 plates from the steel mill to the mold shop, and workers do not need to arrange any reheating or furnace treatment. In the delivery condition, hardness is precisely controlled within 280–325 HB. Strike the corner with a Leeb hardness tester and, once converted, the reading settles steadily at around 30 HRC.

To make the outer tub mold for an 8 kg washing machine, a factory may use a 4.5-ton steel block. Sending that block into a vacuum heat-treatment furnace and heating it to 850°C would consume over 1,200 kWh of industrial electricity.

When a steel block is pulled out of a high-temperature oil bath, uneven internal temperatures generate tensile stresses of several hundred megapascals. Positioning holes that were accurately measured on a coordinate measuring machine can shift by more than 0.5 mm under that stress.

Because 1.2311 is supplied pre-hardened, fitters save 72 hours of reworking hole positions on the grinder. Once the cooling channels have been machined and the cutting oil is wiped off, the steel can be bolted directly onto the injection molding machine platen.

On a CNC mill, the spindle reaches 63 mm in diameter and carries five coated inserts. The operator lowers the safety door, presses start, and the cutter head enters the steel surface at 80 m/s.

The inserts cut grooves 5 mm deep into the steel. The cutting sound is low and steady, without the harsh squeal typical of machining very hard metals. The 1.5% manganese in the steel improves matrix toughness, and the chips curl out smoothly along the flute channels.

The spindle speed is set at 2500 RPM, the feed rate is held at 1200 mm/min, and coolant at 20 bar washes over the tool. The chips come off in neat, regular C-shapes.

A machining center running continuously for 24 hours can consume 150 kg of 1.2311 steel. The scrap collected in the chip cart shows a uniform silvery metallic luster, without the blue-purple burn marks associated with overheating.

Cooling channels are the pathways that keep mold temperature balanced. Deep-hole drilling machines are used to drill holes several hundred millimeters into the steel block. A gun drill only 10 mm in diameter may be required to create a blind hole 600 mm deep through the center of the block.

The drill rotates at 1000 rpm, while cutting oil at 50 kg pressure is ejected from the front to force chips out. Since internal hardness variation in 1.2311 stays within 2 HRC, the drill does not strike hard spots and snap inside the hole.

After 40 hours of continuous milling, inserts show only mild wear, and changing them takes about 15 minutes. With quenched steel hardened to 50 HRC, carbide inserts may chip after only 12 hours of cutting.

The steel mill blows pure argon into the furnace, reducing oxygen content in the molten 1.2311 steel to below 15 ppm. This extremely low oxygen level greatly reduces oxide scale and internal inclusions.

After the mold cavity is cut by machine, the surface is left with tool marks and a roughness of Ra 3.2 μm. A polisher takes a 400-grit ruby oilstone, wets it with kerosene, and pushes in one direction.

The tool marks are gradually leveled. The polishing tools are then changed from oilstone to wooden sticks and finally to pure cotton wheels. Each pass removes less than 0.01 mm of metal from the surface.

By the 30th hour of polishing, surface scratches are no longer visible to the naked eye. A wool felt wheel charged with 3-micron diamond paste spins at 15,000 rpm across the steel surface.

After a full 40 hours of polishing, the roughness remains at Ra 0.15 μm. The cavity surface becomes mirror-bright and clearly reflects the overhead LED lights in the workshop.

Molten ABS at 240°C is then injected into the mold at 120 MPa, and the liquid plastic conforms completely to the smooth metallic cavity surface.

The television housing cools and solidifies after demolding, with a surface as smooth as glass. Reflectivity testing reaches as high as 85%. Workers then apply anti-scratch film and pack the parts into 900 mm cartons for shipment to the assembly workshop.

Practical applications

In the injection molding workshop of an automotive parts plant, a giant molding machine with 3300 tons of clamping force is in operation. Bolted to its base is a bumper mold 2.5 meters long and 1.2 meters wide for front and rear automotive bumpers.

Making this mold consumes 18 tons of 1.2311 steel plate. After the machine tools remove 6 tons of scrap, the remaining cavity contour matches the streamlined curves of the car body.

Modified polypropylene (PP) at 230°C is forced into the wide mold cavity by the screw at 150 MPa.

The molten plastic fills the entire 2.5-meter cavity in just 3.5 seconds. With a thermal conductivity of about 29 W/(m·K), 1.2311 rapidly transfers heat from the plastic to the cooling water channels inside the mold.

Chilled water at 15°C flows through the channels, removing heat and allowing the plastic to harden within 45 seconds. The mold opens and closes continuously, producing 1,200 black bumpers with primer gloss every 24 hours.

In Dongguan, Guangdong, at a home-appliance mold manufacturing base, a forged block of 1.2311 measuring 1500 × 900 × 400 mm sits on a wooden pallet in the workshop.

A fitter uses an oilstone to work the cavity wall and create a fine textured finish. The plastic rear shell of a 65-inch LCD television has extremely strict physical requirements for surface grain.

Etching solution corrodes the steel surface to form an even mesh texture only 0.03 mm deep. Because 1.2311 contains 1.5% manganese, the etched texture develops with very uniform depth under acidic treatment.

· the barrel is loaded with PC+ABS resin pellets

· a dryer blows 90°C hot air for 4 hours to remove moisture

· the mold is packed with dense 12 mm cooling channels

Once the plastic part is injected and demolded, a robotic arm lifts out the TV rear shell with its delicate matte texture. Even after 500,000 cycles, the texture remains clear and is not washed flat by the hard plastic.

The 240-liter green plastic waste bins used in municipal sanitation are also commonly made with 1.2311 molds. The bin body is 950 mm deep, requiring a very long core to be machined.

The steel mill cuts round bars into 1-meter lengths and sends them to the shop. A lathe running at 800 rpm turns the outside surface to create the chamfered form of the inner liner core.

HDPE used for trash bins is extremely tough, and during demolding it can produce as much as 300 kg of frictional pull on the steel surface.

A steel surface maintained at around 30 HRC shows no scratches at all. Even after the mold undergoes thousands of friction cycles per day and three years of intensive production, wear on the outer diameter of the core remains below 0.08 mm.

Children’s building-block toys sold in large supermarkets usually have parts measuring 10–50 mm. A single mold may contain 64 or even 128 identical micro-cavities.

When machining multi-cavity molds, carbide drills on machining centers must produce hundreds of micro vent holes less than 2 mm in diameter in 1.2311 plate.

· spindle speed rises to 25,000 rpm

· feed per cut is adjusted to 0.05 mm

· high-pressure cutting fluid jets from the drill tip to prevent chip clogging

Because the steel is clean and uniform, with no hard inclusions, a small drill can make 300 holes without a single breakage. In one injection shot, 64 colorful building blocks can drop together onto the conveyor below.

The mold block withstands 2000 tons of clamping force, yet the parting surfaces remain perfectly sealed. Even liquid plastic as thin as 0.02 mm cannot flash through the joint.

White wall-mounted air-conditioner front panels are extremely sensitive to dimensional tolerance. On an 850 mm-long plastic part, assembly-line gap tolerance must stay within 0.3 mm.

Internal tensile stress in the steel is extremely low. After scanning a hollowed-out mold measuring 850 mm in length with a coordinate measuring machine, lift at both ends remains within 0.05 mm.

At 210°C, HIPS fills the cavity. The steel’s strong thermal conductivity allows the plastic temperature to drop evenly across the entire cavity, so the finished panel comes out flat and rigid, without warped ends that would fail on the assembly line.

P20

AISI naming

Open an American metallurgy manual from the 1980s and, on page 745, you find one of the earliest steel numbering systems. Back then, Pennsylvania steel plants were producing tens of thousands of tons of metal every day. If workers had to distinguish steels by memorizing chemical compositions alone, sooner or later they would blow up a furnace. The American Iron and Steel Institute, together with several automotive industry associations, developed a short code system that worked almost like license plates.

Steel used specifically for plastic molds was placed in its own category. Engineers wrote the letter P on drawings, taken from the word Plastic. At the time, Detroit automakers were testing large numbers of Nylon 66 plastic parts. Injection molding machines forced molten plastic at 250°C into steel blocks like a syringe injection.

Faced with instantaneous press loads of up to 500 tons, ordinary steel would develop surface cracks before surviving 20,000 cycles. Materials specialists began adding alloy powders to molten steel in different proportions, trying to find the most durable formula. The lab registered hundreds of experimental formulas, each with its own serial number.

· P1 was assigned to a very low-carbon soft steel

· P4 included roughly 5% chromium for corrosion resistance

· P20 was assigned to a formula with 0.35% carbon

· P21 contained about 0.20% vanadium

· P39 was reserved for an extremely expensive high-cobalt alloy

The number 20 does not mean the steel contains twenty percent of anything. It is simply a sequential locker number assigned in order. Workers in a Pittsburgh steel plant received a confidential batching sheet requiring them to control the addition of several chemical elements precisely in a 150-ton heat of boiling molten steel.

The numbers on the spectrometric lab report determined whether that heat could be stamped P20. Carbon had to fall within a very narrow window of 0.28%–0.40%. Too little, and the steel would not survive even 100,000 injection cycles as a mold. Chromium had to be precisely controlled within 1.40%–2.00%.

Workers used an overhead crane to lower a 400 mm thick dark-red forged steel block into a huge oil tank for rapid cooling. After quenching, the block was cut open, and the hardness at the center still had to read above HRC 28. The steel also contained 0.30%–0.55% molybdenum, a gray-black powder worth many times more than scrap steel.

With molybdenum added, the microscopic crystal structure of the steel locked together more tightly. A Detroit machine shop used this same batch to produce a 1.2-meter Ford front bumper mold. When a carbide milling cutter cut the steel at a linear speed of 150 m/min, the chips came off in fine C-shaped curls.

This rugged American letter-and-number naming style was so convenient that it spread worldwide through ocean shipping orders. Traders in Düsseldorf looked at customs paperwork marked P20 and checked it against thick European standards books. Europeans preferred five-digit numerical designations.

Procurement lists often show 1.2311, and those dry numbers follow an extremely rigid classification logic.

· the initial 1 indicates steel rather than non-ferrous metal

· the middle 23 refers to tool steel used for machining

· the ending 11 is its registration number in the European standards system

· the ending 12 requires the addition of about 0.05% sulfur

Steelworkers added sulfur simply to reduce the burden on cutting tools. When machining a television rear-cover mold measuring 800 × 600 mm, sulfur-bearing steel could save nearly 40 hours of cutting time. But when polishers used 1000-grit abrasive paper, sulfides left micron-level black specks in the metal.

Steel with dark spots could not be used for high-gloss LCD television bezels. Mold factories had no choice but to return it and buy a cleaner, original-grade American P20 formula instead. In Asia-Pacific spot warehouses, steel blocks of all sizes sat rusting in stock. Japanese JIS standards did not offer a short equivalent code.

Mitsubishi Heavy Industries machine operators often substituted SCM440 alloy structural steel on drawings. In the metalworking district of Chang’an, Dongguan, workers used a band saw to cut a 500 × 500 mm square block. It was placed on a large digital scale, and the red display settled at 390 kg.

Traders calculated the price at the morning spot rate of RMB 18.5 per kilogram. On the triplicate paperwork, the warehouse clerk did not even bother writing all three letters, casually scribbling “U.S. P steel.” A 5-ton single-girder crane swung the block onto a blue light truck, and in less than two hours it had arrived at a roaring CNC mold shop in Dalang.

Composition and proportions

At a specialty steel plant in Jiangyin, Jiangsu, a 150-ton electric arc furnace roars at full volume. A temperature probe shows the molten steel has already reached 1630°C. Workers in heavy heat-resistant gear push carts of dark alloy fragments and dump them into the boiling furnace mouth.

A lab sampler scoops out a small amount of molten steel with a long-handled ladle. After cooling, the sample is placed into the spark chamber of a direct-reading spectrometer. The machine emits a brief crackling sound, and a string of decimal values for chemical elements immediately appears on the screen. Blending this pot of metal is very much like weighing seasonings by the gram in the kitchen of a fine restaurant.

A single alloy milling cutter can cost RMB 800. If the carbon reading on the screen exceeds 0.42%, machining that steel may chip and break the tool in less than half an hour. The entire 150-ton heat would be scrapped, and even the slag would have to be dumped and remelted.

To keep mold manufacturers from complaining, operators hold the carbon content tightly at around 0.35%. Carbon alone is nowhere near enough to provide the hardness needed to withstand hundreds of thousands of molding impacts. Operators watch the chromium number closely, because that element is what drives the real transformation.

After a solid steel billet 500 mm thick is forged, it is thrown into an oil quench at 850°C for rapid cooling. About 1.5% chromium allows the cooling effect to penetrate deep into the thick metal section. When workers cut the block open, hardness at the center still measures above HRC 28, eliminating the problem of hard outside and soft inside.

The costliest element on the charge sheet is molybdenum. Scrap rebar is worth less than RMB 3 per kilogram, but high-purity ferromolybdenum can approach RMB 400 per kilogram. No matter how carefully the workshop manager counts costs, at least 0.30% molybdenum still has to go into the molten steel.

Remove just 0.1% molybdenum, and the internal crystal lattice becomes coarse and loose at high temperature. A plastics factory in Yuyao, Ningbo once tried to save money with low-grade material lacking molybdenum when making a beach-chair mold. Molten plastic at 220°C repeatedly scoured the cavity, and before the mold even reached 150,000 cycles, a 3 mm crack tore open at one corner.

Silicon and manganese powders are shoveled into the furnace in large quantities. In the 1600°C molten bath, they bind with hidden oxygen in the steel and form a dark-red slag layer nearly 20 cm thick that rises to the top. Operators use a 5-meter slag rake to scrape it into a slag pit, preventing rice-grain-sized pits from appearing after solidification.

Phosphorus and sulfur are tightly controlled impurities on national-standard lab certificates. Their content must be reduced below 0.030%. If excessive impurities are present and the steel meets the violent closing force of a 1000-ton Haitian injection molding machine, a load-bearing block weighing hundreds of kilograms can split into scrap.

Once the alloy powders are fully melted, a large heat-resistant steel pipe is inserted to the bottom of the melt and high-pressure argon is blown in. The bubbles roll through the steel like boiling soup for a full 45 minutes. 1.8% chromium and 0.4% molybdenum are distributed evenly into every microscopic gap between iron atoms.

The red-hot ingots then move to the forging shop. A 3000-ton hydraulic press hammers down on the softened steel at 1150°C. With every blow of several hundred tons, the previously loose internal structure becomes denser. The noise shakes the glass in the control booth, while flakes of oxide scale fly up in sparks nearly 2 meters high.

A few days later, a 13-meter semi-trailer loaded with silver-gray steel plates arrives at a steel market shed in Chang’an, Dongguan. The buyer pulls out a handheld spectrometer shaped like an infrared thermometer, places it against the plate edge, and pulls the trigger. A tiny spark flashes, and the green-on-black display shows Cr: 1.62% and Mo: 0.38%.

Limitations

Beside an 800-ton injection molding machine, a fan-base mold weighing 2.5 tons is removed from service. This mold cost RMB 60,000 to machine and had been running for less than three months.

The drawing called for 300,000 fan bases. But as soon as the machine counter passed 85,000, the molded parts began showing a ring of sharp flash around the edges. The two metal plates, once tightly sealed, had been eroded open into a 0.15 mm gap.

To save RMB 20,000 in material cost, the purchasing department ordered pre-hardened P20. The shop then added 30% glass fiber into the plastic pellets.

The glass-filled Nylon 66 melted in the barrel at 260°C, and the screw rammed the hot slurry full of tiny glass particles into the mold cavity at 120 mm/s. The material as supplied from the steel mill was only about HRC 32.

Against glass fiber, a 32 HRC metal surface behaves like butter rubbed with coarse sandpaper. In less than a month, the metal protrusions that formed the locking tabs on the fan base had worn down by almost 0.2 mm. On the assembly line, operators with electric screwdrivers could no longer drive screws into the misaligned holes.

· for plastic parts containing glass fiber, mold hardness should be raised to at least HRC 48

· if ordinary pre-hardened steel is reheated to 1000°C for hardening, it can distort by 0.5%

· even a dimensional change of 0.1 mm in a steel block weighing hundreds of kilograms can wipe out the entire machining cost of the mold

The same risk applies when processing plastics containing flame retardants. A router-housing factory in Dalingshan, Dongguan received an export order for European fire-test requirements. Instead of ordinary ABS, it switched to acidic PVC.

As soon as the machine started, the PVC, heated to 200°C in the runner, released a sharp acidic odor. That was hydrogen chloride gas produced as the polymer chains broke down.

After just one weekend, the operator lifted the dust cover on Monday morning and found the entire cavity coated with reddish-brown rust. When the surface was wiped with an oily rag, the polished steel was covered in fine corrosion pits.

The steel formula contained less than 2% chromium. Under constant attack from acidic industrial gases, that small amount could not form a complete protective oxide film.

The mold shop had no choice but to spend three times more on S136 stainless steel with more than 13% chromium to handle corrosive plastic. Choose the wrong low-cost material, and a multi-ton mold may not last even one week of production. In the polishing room, the material’s innate defects become even more obvious.

A small manufacturer in Yuyao, Zhejiang took an order for car headlamp covers requiring 90% light transmittance. The polisher worked through three full overnight shifts, going from 600-grit paper all the way to 3000-grit Korean Warrior-brand wet abrasive, even using diamond polishing paste costing hundreds of yuan per tube.

But under a high-intensity inspection light, the metal surface that should have reflected like a mirror was covered with dark, fine streaks. Under a 50× industrial magnifier, the surface was filled with tiny micron-scale pits.

· gases cannot be completely removed from molten steel during smelting, so microscopic porosity remains after solidification

· sulfides and oxides cluster during crystallization into invisible hard particles

· when a high-speed felt wheel sweeps across the surface, it pulls those tiny particles out and leaves miniature crater-like pits

Under SPI mold polishing standards, optical-grade transparent parts must reach SPI A1 flatness. No matter how skilled the polisher is, a steel block with inherent metallurgical porosity can only achieve about a B1 finish at best.

Regional market terms

North American market

In North America, factory drawings often use AISI codes. Grades beginning with P are intended specifically for injection molding. Back in 1950s Detroit, the rush to mass-produce plastic dashboards pushed specialty steel mills to create a formula with 0.35% carbon and 1.70% chromium.

Large steel plates arrived at the shop already in the pre-hardened state. Brinell readings typically fell within 285–311 HBW. Machine shops took the blanks directly to the machine tools, removed the reserved 3 mm machining allowance, and kept deformation to around 0.02 mm.

At the bottom of Ford and GM engineering drawings, you often see ASTM A681-08. Inspectors use a magnifier to check the molybdenum percentage on the certificate. The bottom line is 0.30%—if it falls below that, the ultrasonic flaw detector display turns into a mess of noise.

Before shipping from mold-steel warehouses in the Great Lakes region, large injection molders in North America often focus on these internal cleanliness requirements:

· non-metallic inclusions, thin series A, no more than Class 2.0

· grain size at least Grade 7

· ultrasonic inspection meeting the Ø3.0 mm flat-bottom hole standard

If a polished mold surface looks like orange peel, the lab certificate may show carbon as high as 0.45%. In that case, coarse carbides fill the matrix, and even 3000-grit paper cannot eliminate the tiny pits.

A 2.5-ton bumper mold that had run 150,000 cycles once cracked at a cavity corner. Laboratory transverse impact testing showed only 12 joules. The forge had failed to provide a sufficient forging ratio.

At the Finkl steel plant in Illinois, giant electric arc furnaces roar continuously. 30-ton ingots are loaded into vacuum arc remelting furnaces, where pressure is reduced to 10^-3 Torr, stripping hydrogen from the molten steel until less than 2 ppm remains.

CNC programmers work from toolmaker parameter sheets and do not arbitrarily increase speeds when manganese reaches 1.50%. They stay within figures like these:

· rough milling speed: 120–150 m/min

· feed rate: 0.15–0.25 mm/tooth

· coolant concentration: 8%–10%

In Ontario, Canada, many shops prefer nickel-modified grades. Drawings often specify 0.80%–1.20% nickel. For molds thicker than 400 mm, when the core is cut open and tested, the hardness difference between the surface and center is usually less than 2 HRC.

In California, polishers charge by the hour, often USD 80–120 per hour. Buying a low-sulfur grade with sulfur below 0.03% can reduce the labor required to achieve an SPI-A2 mirror finish by nearly 40%. Buyers are often willing to pay an extra USD 0.50 per pound for better material.

Mold-base shops constantly battle with long cooling channels. A steel plate measuring 800 × 600 mm may require 15 meters of internal water lines. If a gun drill running at 800 rpm drifts by just 0.5 mm, it can break through the adjacent heater-channel wall.

In Texas, some components are molded from high-temperature PC. The barrel temperature reaches 320°C, causing severe thermal cycling on the mold surface. Drawings specify ion nitriding on the cavity surface, with a 0.15 mm nitrided layer reaching 650 HV hardness.

Repairing chipped edges is routine. TIG welders follow American Welding Society procedures with strict limits:

· preheat the entire mold to 250°C

· use ER80S-G filler wire

· keep interpass temperature below 300°C

An Ohio headlamp plant once received a non-standard batch with hardness up to 35 HRC for lamp molds. Spindle load shot up by 25%. Inserts that normally lasted 45 minutes began chipping after 28 minutes. The high-carbon, high-chromium composition simply destroyed the tooling.

Materials manuals list thermal conductivity at room temperature as 30 W/(m·K). With 12 liters/minute of cooling water, a molded part can demold in 22 seconds. But if management wants to push that down to 18 seconds, the mold often has to be hollowed out and fitted with faster-cooling beryllium-copper inserts.

At an electronics housing plant in Boston, production stopped after a coordinate measuring scanner showed the moving platen had sagged 0.08 mm at the center. The quality certificate listed a yield strength of 850 MPa, but after months under 500 tons of clamping force, the plate had still developed a permanent depression.

European market

In Europe, the material field on drawings often reads 1.2311. Under DIN EN ISO 4957, its chemical name is 40CrMnMo7.

When incoming material arrives, inspectors first look for the EN 10204 3.1 certificate. It contains a dense set of lab data. On one batch, carbon may read 0.38% and chromium around 1.90%.

European steel mills commonly market 1.2311 and its close relative 1.2312 together in the same catalog. Before purchasing, the workshop manager checks the drawing’s surface requirement, because the real difference between the two is a small amount of sulfur.

CNC operators love 1.2312. With that extra sulfur, chips break into 5 mm segments, and when the spindle runs at 1200 rpm all day, tooling costs can drop by more than EUR 100.

But a factory in Munich making BMW interior components learned the downside. Workers used 1.2312 for texturing to VDI 3400 No. 27. Once the acid etchant was applied, sulfide inclusions were corroded away, leaving 0.05 mm pits across the molded surface.

Ultrasonic testing is taken very seriously in Europe. Operators scan according to SEP 1921, and on 1.2311 blocks thicker than 500 mm, internal defect levels must stay within the D/d acceptance line.

European steel mills also adjust hardness before shipping. Using a 10 mm tungsten carbide ball under a 3000 kg load, Brinell hardness is converted and kept steadily within 280–325 HB.

Audi mold designers routinely consult handbooks for cooling calculations. At 20°C, the thermal conductivity of 1.2311 is 34 W/(m·K). After prolonged injection molding raises the mold to 200°C, that value gradually falls to 31 W/(m·K).

In Dortmund forging shops, 5000-ton hydraulic presses hammer red-hot ingots. To fully break down coarse grains, the forging ratio is pushed above 4:1. Rough billets weighing 15 tons are repeatedly drawn out and deformed.

Tempering cycles are long. To relieve machining stresses after rough cutting, the furnace is held at 600°C for a full 4 hours, then cooled in a tightly controlled program at 20°C per hour.

A pre-hardened 1.2311 block measuring 1 × 1 meter and weighing 2 tons, complete with the original ultrasonic report, is typically priced at around EUR 2.80–3.50 per kilogram.

In Frankfurt, welders wearing face shields repair broken mold corners. Drawings call for TIG welding, and the work order specifies preheating the mold to 250°C. The filler rod must be 1.2311-IG, and the machine current is set to 120 A.

For plastic parts containing more than 30% glass fiber, Europeans often use gas nitriding. A finished 1.2311 cavity may stay in the nitriding furnace for 48 hours, producing a 0.15 mm nitrided layer with a hardness up to 650 HV1.

On a DMG MORI five-axis machining center, cutting parameters remain conservative. Rough cutting of large 1.2311 stock uses a depth of cut around 3 mm. If manganese reaches 1.50%, feed is reduced to about 800 mm/min.

In Italy, washing-machine drum mold makers often check the parting line. After a 1.2311 master plate cycles continuously for half a year on a 400-ton molding machine, the parting edge may collapse by 0.04 mm.

China market

Under GB/T 1299-2014, Chinese engineers commonly refer to the material as 3Cr2Mo. The “3” indicates a carbon content between 0.28% and 0.40%, while “Cr2” indicates roughly 1.40%–2.00% chromium.

In Chang’an, Dongguan, specialty steel dealers handle hundreds of tons of trucks every day. Pre-hardened plate from Fushun Special Steel stamped FS usually wholesales for RMB 12–15 per kilogram, while imported 1.2311 displayed on nearby racks may be priced above RMB 28 per kilogram.

Veteran inspectors in workshops have an unwritten rule: when ordering plate thicker than 300 mm, always check the original mill’s ultrasonic testing report and never rely only on the trader’s verbal hardness claim.

Automotive mold plants in Huangyan often handle Great Wall bumper orders. When 5–6 ton 3Cr2Mo blanks are cut open, core hardness may fall. Chinese national standards allow a maximum difference of 3 HRC between surface and core. Beyond that, polishing creates visible light-and-dark patches on the mold surface.

Operators in Kunshan machine shops immediately adjust FANUC feed parameters when working with domestic plate. Based on experience with cutting resistance, senior machinists write practical shop-floor values on whiteboards for workers to follow:

· face-mill spindle speed kept to 600 rpm

· depth of cut for rough and finish machining limited to 0.8 mm

· coolant flow increased to 15 L/min to wash chips away

For textured home-appliance housings, steel is sent to etching shops. These plates face very high demands. If deoxidation during steelmaking is poor and non-metallic inclusion ratings exceed 2.5, nitric-acid texturing becomes uneven, and scrapping a single plate weighing several hundred kilograms can cost tens of thousands of yuan.

In Shenzhen, phone-case subcontractors run Sodick EDM machines every day. In rough settings, discharge current may be raised to 45 A as the copper electrode burns into the cavity. A bright recast layer forms on the EDM surface, and hardness can reach 55 HRC.

Before polishing an EDM-finished mold insert, apprentices must file off that 0.02 mm white layer. Leave it in place, and even two days with 3000-grit paper will not produce a mirror finish.

Baosteel Special Steel uses vacuum degassing for such material. Molten steel from the electric furnace goes into a VD unit, reducing harmful oxygen below 15 ppm. With cleaner steel, transparent PC headlamp covers are less likely to develop orange peel.

Slow wire cutting is often used for thick plate because deformation is a concern. On a Su Sanguang machine, 0.2 mm molybdenum wire is threaded to cut a 150 mm thick insert. With high-pressure water flushing and a cutting speed of 40 mm²/hour, the diagonal error after cutting can stay under 0.01 mm.

Household appliance factories molding glass-fiber-reinforced plastics see very fast mold wear. Drawings often require the cavity surface to be sent for soft gas nitriding. After 6 hours at 570°C, a 0.1 mm nitrided layer is formed, extending mold life by another 50,000 cycles.

TIG welders repairing collapsed corners need a very steady hand. Domestic 3Cr2Mo has a relatively high carbon equivalent, so cold welding cracks easily. Workshop boards typically list strict repair parameters:

· preheat the repair zone to 200°C with an oxy-acetylene torch

· use H08Cr2MoA filler wire to match the grade

· after welding, cover the area with thick asbestos cloth for 2 hours of heat retention

In Guangdong, dealers often mix grade names. When customers ask for 3Cr2Mo, sales staff frequently suggest paying a little more for 718H. With about 1% more nickel, 718H offers better hardenability and can reach 33–37 HRC.

Under GB/T 17394, hardness is measured at six points, the highest and lowest values are discarded, and the average is converted to Rockwell hardness. Only values within 28–32 HRC are accepted for sign-off.

When using low-cost Walter carbide inserts costing only a few dozen yuan each, operators watch the flank wear carefully. Once the wear band exceeds 0.3 mm, the insert must be indexed immediately, or continued cutting may damage the machining center spindle bearings.

Heavy semi-trailers from Daye Special Steel haul hot-rolled wide plate southward. Before shipping, plate thicker than 50 mm is stress-relief annealed in a gas furnace. At a furnace temperature of 650°C, the cut strips come out without bending or warping.

In Taizhou’s mold market, inspectors cut a 10 mm thick sample on a saw and boil it in 50% hot hydrochloric acid for half an hour. Under GB/T 226 low-magnification inspection, if central porosity exceeds Grade 2, the entire batch is returned to the trader.

When molding ABS engineering plastic, the mold temperature controller keeps the mold at 60°C. Cavity shrinkage allowance is left at five thousandths. For a television rear-cover mold with a diagonal size of 2 meters, fitters typically require machinists to leave 0.05 mm of closing allowance.