P20 Steel or 1.2738 | Mold Size, Toughness, Polishability

P20 is suitable for small to medium-sized molds, with a hardness of HRC 28–32 and good polishability.

1.2738 is a modified P20 grade with approximately 0.9–1.2% Ni added.

It offers better through-hardening in thick and large cross-sections, making it more suitable for large molds.

Material selection should be based on mold size and mirror-finish requirements to ensure both toughness and structural uniformity.

Mold Size

Small to Medium-Sized Molds

A solid block of P20 steel with a thickness of 250 mm to 400 mm, sitting on the floor of a machining shop, looks like a large rectangular iron chest. Taking a common size of 600 mm long, 400 mm wide, and 300 mm thick, the weight on the scale comes out to roughly 560 kg.

The single-arm crane in the factory is rated for 2 tons. The crane operator fits two M20 lifting eye bolts, and one person can steadily place the block onto the cast-iron worktable of the CNC machine in about 15 minutes. The operator sets the spindle speed to 10,000 rpm and installs a 12 mm four-flute carbide end mill.

The tool tip cuts 0.5 mm into the surface of the steel block and advances forward at a feed rate of 1,500 mm per minute.

· Leave a machining allowance of 0.3 mm per side during rough milling

· Set the finishing toolpath step-over precisely to 0.15 mm

· The roughness tester reads Ra 1.6 on the bottom surface of the cavity

· The emulsion concentration in the coolant tank is maintained at 8%

A steel block with a volume just over 0.1 cubic meter cools very quickly in the quenching tank before delivery. In 25 minutes, the core temperature can drop to room temperature. If a 300 mm-thick offcut is sectioned and examined under a metallographic microscope at 500× magnification, the grain size rating from the surface down to a depth of 150 mm can still reach grade 8 or above.

Under the microscope, the pearlite and ferrite structure is packed together as tightly as fine sand. The brightness of the mold cavity surface ultimately depends on patient hand polishing. The polisher holds a high-intensity flashlight in the left hand and a 200-grit oilstone in the right, pushing and grinding firmly in one consistent direction.

After two hours of back-and-forth grinding, it is just enough to remove the 0.01 mm-deep tool marks left by CNC milling. Then 400-grit and 600-grit imported wet sandpaper are used for cross-direction polishing. Finishing a metal cavity about the size of a mobile phone case takes a full six hours.

· Maintain about 2 kg of downward pressure when pushing the oilstone

· Average sandpaper consumption is 3 sheets per square meter

· The polishing paste is mixed with 3-micron diamond powder

· The wool wheel runs at 1,500 rpm against the metal surface

The mold plates behind everyday products such as TV remote control covers and wireless router housings are rarely thicker than 350 mm. The injection molding shop typically pairs them with small to medium injection molding machines rated at 150 to 350 tons clamping force. Molten ABS at 220°C behaves inside the barrel like a high-temperature, high-pressure water jet.

Under a pressure of 80 MPa, the plastic melt fills the entire metal cavity in a fraction of a second. The instantaneous expansion tension borne by the cavity wall reaches 600 kg per square centimeter. Dozens of intersecting cooling channels with an 8 mm diameter are drilled throughout the steel block.

The industrial chiller pumps purified water at 15°C into the cooling channels at a flow rate of 15 liters per minute. The circulating water forcefully removes the heat released by the 220°C plastic, keeping the mold surface operating temperature firmly within the 60°C to 80°C range.

For a router housing mold with a single cavity volume of 15 cm³, the machine can complete the full cycle—clamping, injection, holding pressure, and opening—in just 8 seconds. Running nonstop for 24 hours, the injection molding machine strikes the parting surface of the steel block 10,800 times a day.

The 300 MPa yield strength of P20 in the prehardened delivery condition withstands these repeated impacts. After the mold has been in continuous production for half a year, it is removed from the machine and fully rechecked.

· Vent grooves at the edges must be repaired by welding if wear exceeds 0.02 mm

· The clearance of the load-bearing guide pillars may increase to 0.05 mm

· If the gate area is enlarged by 0.1 mm from plastic erosion, the insert must be replaced

· The ejector pin holes at the bottom may show an ovality deviation of 0.03 mm

For plastic shell parts with overall dimensions within 200 mm, the 3D dimensional tolerance measured by a CMM ruby probe is controlled within ±0.05 mm. After producing 300,000 parts, the cavity is checked again on the machine, and the deformation at several designated monitoring points remains below 0.02 mm.

When making a 16-cavity toy brick mold for Lego-style blocks, the drawings impose extremely strict limits on machining deformation. The machining center removes 60% of the steel block’s internal volume, leaving only 15 mm-thick walls between adjacent cavities.

The mold maker uses a low-speed wire-cut EDM machine to cut insert slots only 0.8 mm wide. The extremely fine molybdenum wire produces end-face perpendicularity deviation of less than 0.005 mm. In P20 with a thickness below 400 mm, residual stress is very low, so even after more than half of the metal is removed, the block does not warp or bend.

Assembling a dual-cavity phone case mold involves nearly a hundred metal components, including the support plate and ejector plate. A large core-retaining plate 1,000 mm long, after surface grinding, holds parallelism at both ends within 0.02 mm. After assembly, the fitter checks the shut-off gap with feeler gauges.

A 0.01 mm precision steel feeler gauge cannot be inserted into the gap between the two steel plates. The upper and lower mold halves fit perfectly, and the plastic parts produced under high injection pressure come out with clean, smooth edges and no flash at all.

The thermometer and hygrometer on the wall of the injection molding shop stay around 25°C and 50% relative humidity year-round. During line changes and shutdown maintenance, the operator sprays a 0.05 mm anti-rust oil film into the cavity. Then a forklift operator lifts the mold with a 2-ton electric forklift and moves it into a constant-temperature, constant-humidity warehouse.

Large / Ultra-Large Molds

When the thickness of a steel block exceeds 600 mm, or even reaches 1,000 mm, it stands in the machining shop like a dark gray metal wall. For the rear housing mold of an 85-inch LCD TV, a single block of 1.2738 steel may measure 2,200 mm by 1,400 mm. The total weight of the solid metal easily climbs to around 15 tons.

The crane operator climbs into a twin-girder overhead crane three meters above the ground and lowers two reinforced steel cables rated for 20 tons. Four fitters work together to screw four giant M48 lifting eye nuts into the prefabricated threaded holes in the side of the block. With a deep mechanical rumble, the crane moves the massive steel block onto the table of a five-axis gantry machining center at a safety speed of less than 2 meters per minute.

The machine spindle is fitted with an 80 mm inserted-face milling cutter, and six coated carbide inserts cut into the steel surface one after another. The depth of cut is set to 2 mm, and the spindle speed is reduced to 800 rpm. At a feed rate of 500 mm per minute, fingernail-sized dark purple chips pound against the machine guard like heavy rain. After 48 hours of continuous rough milling, the removed chips fill three sheet-metal bins, each with a volume of 1.5 cubic meters.

The CNC tool removes material downward to form an 800 mm-deep mold cavity. In ordinary steel, once machining reaches such extreme depths, the hardness at the center can drop below 20 HRC, becoming as soft as common copper plate. In 1.2738, the 1.05% nickel in the alloy starts to show its value. Hardness testing on the bottom of an 850 mm-thick section still gives a stable core hardness of 29 HRC.

When polishing the deepest areas of an automotive bumper mold, the polisher has to lean half of the body into a metal recess more than one meter wide. The very small hardness difference between the 32 HRC surface and the 29 HRC core ensures that the entire curved surface feels nearly uniform during polishing. Following the curvature of the surface, workers use 800-grit sandpaper for more than 30 hours, producing a large-area etched texture with fully consistent diffuse reflection across the same face.

To cool a metal block weighing well over ten tons, the deep-hole drilling machine must create cooling channels with a combined length of more than 120 meters. A 16 mm gun drill penetrates two meters into the steel block at only 20 mm per minute. A high-pressure oil pump forces cutting oil at 6 MPa into the bottom of the hole, flushing chips out through the V-groove of the drill, while drill wear under high friction remains below 0.1 mm.

Ultra-large molds are matched with massive double-arm injection molding machines with clamping force as high as 3,300 tons. When two mold halves weighing 18 tons each close together, the hydraulic system applies more than 3,000 tons of clamping force across the contact surface. Molten polypropylene shoots from the hot runner nozzle and races through a 15 mm main runner, filling a 2-meter-long cavity in 3 seconds.

The plastic melt builds up a maximum holding pressure of 45 MPa deep inside the cavity. If the steel does not have adequate core hardness, repeated high-pressure impact over hundreds of thousands of cycles will cause fatigue yield deformation of more than 0.5 mm in the cavity ribs. The 980 MPa tensile strength of 1.2738 firmly resists this internal expansion stress. The chiller supplies 30 cubic meters of 5°C chilled water into the mold every hour.

Large plastic parts place completely different rigid requirements on mold size and molding equipment parameters. When scheduling large-part production, the shop planner allocates space and resources according to a specific equipment matching list.

The production cycle for an automotive front bumper typically runs between 55 and 65 seconds. A robot arm reaches into the opened mold and takes 4 seconds to steadily pick out the 1.8-meter-long molded plastic part. The mold guide pillars are as large as 80 mm in diameter, with a chromium-plated surface layer 0.03 mm thick. Even after more than a thousand cycles of sliding friction per day, the pillar surface finish remains at Ra 0.4 μm.

During tryout in the mold assembly shop, several 0.02 mm-thick red lead contact papers are placed on the lower mold surface. After the upper mold is pressed down and lifted, the fitter evaluates the pressure distribution across the contact area based on the red transfer marks. The red marks must cover at least 85% of the mating surface uniformly. Where localized uneven pressure appears, the worker uses a pneumatic angle grinder to lightly remove a 0.01 mm layer of metal.

Toughness

What Is Toughness?

A shop crane lifts a huge steel mold weighing 5 tons. If the lifting ring unexpectedly disengages when the mold is only 10 cm above the concrete floor, the falling mass generates an instantaneous impact force of 1,500 Gs. A brittle steel plate may split into two on the spot, while an excellent mold steel block can remain intact externally.

In the quality lab, there is a heavy pendulum impact tester as tall as a three-story building. The operator raises a pendulum weighing dozens of kilograms to a height of 1.5 meters and releases it. A standard steel specimen measuring 10 × 10 × 55 mm is fixed in the slot at the bottom. A precise milling cutter has already machined a 2 mm-deep V-notch on the back of the specimen.

After the pendulum crashes through the specimen and swings through to the other side, the dial records the energy absorbed before fracture. Low-quality steel locks the pointer at 4 joules. Fully heat-treated 1.2738 can drive the pointer steadily to 18 joules.

A worker places a thick steel plate on a CNC milling machine to cut a 400 mm-deep plastic barrel cavity. The carbide tool removes metal at 3,000 rpm, leaving a standard 90-degree inside corner at the bottom of the cavity. After machining stress is released from the huge steel block, that sharp corner becomes the area under the greatest stress concentration.

The melted plastic reaches 220°C. The injection screw forces it into the mold at an extremely high pressure of 150 MPa. Three times every minute, the hot, high-pressure melt violently strikes that 90-degree steel corner. The thick steel wall expands outward by 0.05 mm—far too little to be seen by the naked eye.

If the steel has poor resistance to tearing and deformation, it cannot withstand tens of thousands of such cycles. A tiny crack only 0.02 mm wide opens at the root of the inside corner. After 10,000 molding cycles, that internal crack—originally finer than a strand of hair—silently propagates 5 mm deep into the steel.

The mold interior is filled with a network of cooling channels. Industrial cooling water at 15°C rushes through drilled passages 12 mm in diameter. Every 20 seconds, the damaged steel is alternately shocked by 220°C plastic heat and 15°C cooling water. Once the internal crack extends into a cooling channel, water leaks instantly into the cavity and the molded products are completely ruined.

If you open an internal mechanical testing report from a specialty steel mill—documents rarely circulated outside—the numbers clearly quantify the material’s resistance to cracking. Samples taken from different positions in the same large steel block can show dramatically different results under pendulum impact testing.

On a 1,200-ton injection molding machine, the two heavy doors lock shut with force. The four large tie bars are stretched by 2 mm. The contact faces between the two mold halves are subjected to extremely high cyclic compressive loads. A backing plate that tested at only 5 joules can crack diagonally like split firewood after just one week on the machine.

Bend an ordinary 3 mm steel wire back and forth by hand. A poor-quality wire that is hard but brittle will snap on the third bend. A properly processed high-quality wire becomes noticeably hot at the bend point and can withstand more than twenty repeated folds. The metal grains inside it slide and cooperate efficiently under severe distortion.

In a German steel mill, a three-story heat treatment furnace is precisely heated to 860°C. The massive steel block stays in the furnace for four full hours so carbon atoms can distribute evenly throughout. The red-hot block is then lifted by crane and dropped into a special oil tank, reducing its temperature to 60°C within 10 minutes. The atomic arrangement is locked into place, while still retaining a measure of deformability.

An automotive instrument panel front-shell mold may cost RMB 300,000. The back side contains as many as 45 plastic assembly clips. The slender steel sections that form these clips are as thin as 2.5 mm at their narrowest point. Each time the ejector system pushes the large molded part out, it applies 300 kg of force around these extremely thin steel sections.

Under strong ejection force, the thin steel core may deflect sideways by 0.01 mm. If the steel’s impact value is 20 joules, it can elastically recover and survive 500,000 violent ejection cycles. If replaced with a cheap scrap steel grade rated at only 6 joules, the thin core can snap off at the root on the morning of the third day of trial production.

The tungsten argon arc welding torch instantly generates 2,000°C heat inside a 5 mm-wide crack. The surrounding cold steel rapidly absorbs that heat. The small droplet of filler metal cools so quickly that its hardness surges to a destructive 55 HRC.

The welder then uses a butane flame torch to preheat and temper the area around the hard weld scar. The surface is held at about 400°C for two hours. It is better to sacrifice 5 HRC of surface hardness than to lose 10 joules of impact toughness needed to survive the next 100,000 mold cycles.

P20 vs 1.2738

When the two massive mold plates of an injection molding machine close, they can generate an impact force of 600 tons in an instant. The crystal structure inside the steel has to withstand enormous compressive stress. P20 produced by the mill usually contains 0.38% to 0.42% carbon. Before delivery, the steel block is heated through at 860°C and then annealed at 550°C.

The P20 blocks people usually purchase are generally no thicker than 250 mm. A hardness tester on the surface gives a stable reading of 30 HRC. Ultrasonic inspection shows no coarse grains inside. At room temperature, a standard impact specimen cut from it can absorb 20 joules before fracture.

A mold factory in Guangdong once accepted an order for a 65-inch TV rear housing mold and bought a P20 steel block 550 mm thick. Once that huge block was dropped into the quenching oil tank, the surface temperature plunged to 200°C within 3 minutes. But at a depth of 250 mm from the surface, the core temperature still remained at 600°C.

Because the center cooled too slowly, the austenite transformed into pearlite. The structure coarsened, making the steel softer and more brittle. A sample cut by EDM showed the core hardness had dropped below 25 HRC. Another impact test showed that the absorbed energy had fallen to only 8 joules.

The mold for a large TV rear shell contains dense arrays of long, narrow cooling ribs around 30 mm deep. In every injection cycle, molten plastic is forced into the mold under a pressure of 120 MPa. Steel with degraded internal quality cannot withstand it, and micron-scale cracks quickly appear at the roots of the ribs. After 50,000 cycles, the mold may suffer large-area edge chipping and failure.

When the internal structure of a large steel block has gone wrong, the machining shop can see the signs immediately:

· Cutting insert wear doubles during milling

· Polishing leaves persistent orange-peel patches on the surface

· The drill bit wanders by 3 mm during deep-hole drilling

· Spiderweb-like cracks appear in EDM areas

When producing 1.2738, the steel mill adds ferronickel alloy to the melt. Spectrometer analysis shows the nickel content in this batch at 1.05%. Nickel atoms, with an atomic radius of 1.24 Å, fit into positions within the iron lattice.

With nickel added, the cooling rate required for hardening is reduced by about 40%. A 600 mm-thick giant block of 1.2738 dropped into water shows a noticeably steeper cooling curve at the center on the computer display. The deepest core successfully transforms into fine bainite and martensite.

The inspection certificate issued by ThyssenKrupp for 1.2738 shows excellent figures. Measured at the exact center of a 600 mm-thick section, the hardness remains around 31 HRC. A specimen cut from the same position can still withstand 18 joules of impact. Under an electron microscope, the fracture surface is covered with dimple-like depressions.

In one machining shop, a single 8-ton block of 1.2738 is being used to machine an automotive front bumper mold. A large gantry mill fitted with a 160 mm face cutter begins roughing. It removes 600 cubic centimeters of chips per minute, and the entire block machines without hitting a single hard spot. Because the hardness is uniform throughout the material, tool consumption stays low.

Each bumper takes 80 seconds to mold. The mold temperature control system alternately runs 85°C hot water and 20°C cold water through the pipes. The mold surface expands with heat and contracts with cooling, with a temperature difference of 65°C. 1.2738 resists thermal cycling well and does not develop turtle-shell-like heat checking even after prolonged production.

Experienced mold makers follow practical machining rules for different material thicknesses:

· Use a 20 mm carbide milling cutter for plates under 200 mm thick

· For 300 mm-thick steel blocks, cut only 0.15 mm per pass

· For drilling 500 mm-deep holes, reduce spindle speed to 600 rpm

· Leave 0.3 mm allowance in the corners of large molds for fine finishing

After running day and night for 300,000 molding cycles, the mating faces of the two mold halves have withstood a total compressive load of 24 million tons. The inspector checks the most highly stressed areas with ultrasonic equipment as specified. In this piece of 1.2738, not a single fatigue crack 0.1 mm long can be found.

If the drawing changes, part of the mold may need to be repaired by welding. The TIG welder is set to 150 amps, melting filler wire onto the surface. Ordinary P20 is more likely to harden excessively or crack around the heat-affected zone. The small amount of nickel in 1.2738 makes the steel much safer under high-temperature welding conditions.

After welding, the repair area is covered tightly with an insulating asbestos blanket and allowed to cool slowly. When the fitter reshapes the welded area with a steel file, it feels very similar to filing the original steel. Close visual inspection shows no cracks at all along the weld edge. A repair originally scheduled for 72 hours is completed in 48.

Polishability

Addition of Nickel

In the steel mill, the large furnace is heated to 1,600°C, and workers add precisely weighed pure nickel plates into the glowing molten steel. In the composition of 1.2738, the nickel content is strictly controlled between 0.90% and 1.20%. This addition raises the purchase cost per ton of steel by about 20% to 30%.

Take ordinary 1.2311 steel, which is often compared alongside it. Its composition contains no nickel at all. When a large steel block over 400 mm thick is heated red-hot and dropped into oil, the oil cannot remove heat from the center quickly enough. A hardness tester may read 30 HRC on the surface, but the value falls below 22 HRC toward the center.

Adding about 1% nickel fundamentally changes the internal transformation rate during cooling. During quenching from 860°C, nickel slows down the conversion of the high-temperature structure into softer phases. Even if the deepest part of the steel cools at only 15°C per minute, it can still transform into a dense, strong hard structure.

When a flaw detector scans a 15-ton steel block one meter thick for an automotive bumper mold, the waveform on the screen remains smooth and stable. From the outer skin to the center half a meter deep, the hardness fluctuates only within the narrow range of 28 to 32 HRC.

For an experienced polisher, steel with sufficient nickel content feels noticeably different during rough grinding:

· 800-grit sandpaper produces evenly distributed resistance

· A 200-grit oilstone pushes forward without slipping

· The polishing machine handle hardly vibrates at 500 rpm

· The grinding wheel wears at a steady, predictable rate

· The surface temperature of the steel does not suddenly spike

Under 500× magnification, cheap steel without nickel shows patches of hard and soft areas across the surface. After dozens of passes with 3000-grit ultra-fine sandpaper, the softer zones are gouged away more quickly. Under a flashlight, the reflective surface is covered with dense tiny pits less than 1 mm across—what polishers call orange peel.

Nickel acts like glue inside the iron lattice, holding the internal framework together. When polishing progresses to 3-micron or even 1-micron diamond paste, the high-density wool wheel spins rapidly over the steel. Because the hardness is uniform, the surface can withstand the extremely fine abrasive action steadily, and the roughness can be reduced to around 0.05 μm.

Combined with vacuum degassing outside the furnace, nickel helps double the cleanliness of the molten steel. The chemical analysis is clear: sulfur, which damages polished surfaces, is held below 0.005%, and phosphorus below 0.015%.

Under ASTM E45 cleanliness testing, nickel-containing 1.2738 produces excellent laboratory results:

· Sulfide inclusions below 1.5

· Alumina inclusions below 1.0

· Silicate inclusions below 1.5

· Globular oxides below 1.0

When nickel-containing steel is polished to SPI A-2 level, it can be used to produce 65-inch TV housings with a glossy dark surface on the back. Shop production records show that with material of uniform hardness inside and out, nearly 40 hours of polishing time can be saved on a large mold with a surface area of 2 square meters.

For plastic parts requiring extremely high transparency, the reflectivity must approach the SPI A-1 absolute mirror level with roughness below 0.02 μm. With steel containing about 1% nickel, when exposed to extremely fine, high-speed mechanical friction, the 32 HRC surface can still suffer slight deformation. Under a high-intensity flashlight, a faint white haze can appear on the polished surface.

When making high-transparency PC lenses for automotive headlamps, the steel grade on the purchase order must be changed. Either choose NAK80, with nickel content around 3% and delivery hardness of 40 HRC, or switch to 1.2083 stainless steel, vacuum heat-treated at 1,020°C and hardened above 50 HRC. Those grades are hard enough for that job.

Sand Holes & Pitting

In the polishing shop, a master polisher spends two full days working over a 1.5-meter-wide automotive door panel mold using wool felt charged with 3-micron diamond paste. After wiping the reflective surface clean with alcohol, two or three tiny black spots suddenly appear on the steel. When checked with the micrometer head of a caliper, each spot measures around 0.05 mm in diameter. In shop language, these are called sand holes.

Just a few tiny defects—smaller than sesame seeds—can put a 3-ton 1.2738 steel block at risk of being scrapped. On the corresponding positions of the ABS plastic parts, they will create sharply reflective raised marks. The polisher has to throw aside the 6000-grit cloth and go all the way back to 800-grit sandpaper to remove another full layer across the whole surface.

These black spots are not external dust. They are native inclusions buried inside the steel. Steelworkers call them non-metallic inclusions, mainly sulfur and oxygen that were not fully removed from the molten steel. At 1,600°C, sulfur combines with manganese to form soft manganese sulfides, while oxygen combines with aluminum to form hard alumina particles.

In low-cost ordinary 1.2311 steel, the sulfur content on the certificate often stays above 0.02%. When the polishing wheel runs across the steel at 1,200 rpm, these very hard alumina particles act like stones buried in mud. As the surrounding steel matrix—only around 30 HRC—is ground away, the particles loosen and fall out, leaving behind a pit about 0.02 mm deep.

Before high-quality 1.2738 leaves the mill, the molten steel undergoes full vacuum degassing inside a massive vacuum vessel. Fifty tons of high-temperature steel are held in a sealed environment at only 1 Torr while being violently agitated. Argon is blown in from the bottom, carrying excess hydrogen, oxygen, and various impurities upward in bubbling streams.

After this vacuum degassing (VD) process, the impurity level in 1.2738 is greatly reduced. Sulfur is cut to below 0.005%, and total oxygen is controlled within 15 ppm. The size of the remaining particles shrinks sharply, with cross-sectional diameters below 2 microns.

Below is a comparison list based on ASTM E45 inclusion ratings:

Polishers are particularly frustrated by Type B alumina inclusions. With a Mohs hardness of 9, they are only slightly softer than natural diamond. These micron-sized hard particles are embedded in 1.2738 steel with a hardness of only about 32 HRC. The fine abrasive particles in polishing paste cannot cut them, and the lateral force generated by high-speed friction simply tears them out of the steel.

The moment an inclusion falls out, the wool wheel carries that extremely hard particle wildly across the polished surface. A mirror finish that had just reached Ra 0.1 μm can instantly be ruined by a deep groove 7 mm long. One rough scratch can wipe out half a shift of polishing work.

Repairing a visible sand hole comes at a high cost. The factory must bring in a precision TIG welder, set a low current of 60 amps, and use a 0.4 mm filler wire with the exact same chemistry as 1.2738 to carefully place a tiny droplet of molten metal into the pit.

After welding, the rice-grain-sized repair spot cools rapidly, and the local hardness rises above 45 HRC. The polisher then has to level the protrusion with a carbide rotary burr, followed by 400-, 600-, and 800-grit oilstones in sequence. Just flattening this one repair point can consume three hours of labor.

If you calculate the economics of mold repair, the additional downstream cost caused by impurity-rich low-cost steel becomes obvious:

For a large automotive instrument panel mold, spending an extra ten or twenty thousand yuan on better steel at the beginning is insignificant compared with the losses caused later by filling a few micron-sized pits and shutting down injection machines weighing hundreds of tons.

Using a 100× portable microscope, the polisher examines the surface of a polished 1.2738 mold face. The field of view shows a smooth, even gray ripple pattern—the microscopic scratch lines left by 8000-grit ultra-fine polishing paste.

Because the steel mill invested real effort in vacuum degassing and impurity removal, the internal structure of 1.2738 is clean and pure. The fine particles that make polishers curse ordinary P20 are largely removed in the vacuum vessel.

Prehardening

At the steel mill, a large crane lifts a 15-ton 1.2738 bloom and slowly feeds it into a furnace heated to 860°C. After the red-hot block is quenched, it is placed into a tempering furnace at over 500°C for more than ten hours. Before shipment, the inspector strikes dozens of points on the steel surface with a hardness tester, and every reading falls within the range of 28 to 32 HRC.

The buyer pays for steel that already comes with hardness and can take it straight back to the shop without sending it out for heat treatment. In the mold industry, this advance heat treatment is called prehardening.

Annealed soft steel is extremely soft, with a measured hardness of only about 15 HRC. In the shop, a CNC machining center can cut this soft material at spindle speeds up to 2,000 rpm. Sharp tools bite into it like cutting tofu, and nearly half a ton of curled chips can be removed from the cavity in a single day. Machining is exceptionally smooth.

But once the soft mold has been fully machined and then sent to heat treatment, the thermal expansion and contraction can distort the whole structure. A TV housing mold frame 1.2 meters long may warp by 2 mm at the corners after rapid cooling in the quench tank. Back at the assembly bench, even heavy hammering cannot force the guide pillars into misaligned guide bushes.

When roughing prehardened 1.2738 at 30 HRC, machining parameters have to be adjusted entirely. The machine operator lowers the spindle speed to 800 rpm. With a 30 mm carbide cutter, less than 500 grams of fine chips can be removed per minute from the steel surface.

Heavy roughing on hardened material places a much greater burden on the shop:

· Insert wear is rapid, and the inserts must be replaced after about 4 hours of continuous cutting

· Coolant flow must run at full volume to flush away red-hot chips reaching 300°C

· The machine spindle is subjected to strong vibration, and the motor overload protection light flashes frequently

· Every additional millimeter of cutting depth increases power consumption by 15%

Even so, once a high-precision machine finishes the cavity within a 0.01 mm tolerance and the mold goes off to polishing and assembly, one problematic step has been eliminated: there is no need to send it into a furnace and risk distortion. The mold geometry stays fixed and stable like stone.

A 2.5-ton automotive bumper mold can go from rough machining on the machine tool all the way to polished delivery in a schedule firmly fixed at 35 days. There is no disruption from rework caused by heat treatment deformation.

Once hardness exceeds 40 HRC, ordinary coated carbide end mills start chipping heavily at the first contact. For glass-fiber-reinforced plastic injection molds requiring hardness above 50 HRC, the shop will usually mill the outer shape in soft material first, harden it afterward, and then finish it with expensive low-speed wire EDM at a fine stepping increment of 0.005 mm.

The physical condition of 1.2738 is deliberately kept within the narrow band of 28 to 32 HRC. Standard lathes fitted with titanium-coated carbide inserts can still machine it. Once the programmed cycle is completed, the cut surface does not show deep tearing marks. A fitter using a diamond file with full force can still remove a 0.2 mm-thick sharp burr from the edge.

The more than ten hours spent in the tempering furnace before delivery are extremely important. At 600°C, the internal stress locked in during quenching is released completely. When a wire EDM machine cuts a straight 200 mm slit through the middle of a steel block half a meter thick, the two sides of the cut remain perfectly still. A feeler gauge inserted into the gap shows no dimensional deviation as large as 0.05 mm.

When the factory owner does the math carefully, prehardening eliminates several additional costs:

· It saves the external quenching fee of RMB 3 per kilogram

· It removes the transport cost of moving 3-ton steel blocks back and forth on rented flatbed trucks

· It cuts out the grinding hours needed to clean off the blackened surface after heat treatment

· It avoids the risk of mold cracking and the need to spend tens of thousands more on replacement steel

When a polishing wheel charged with 3000-grit diamond abrasive is pressed against the surface at high speed, a 30 HRC steel surface can still withstand the continuous action of fine particles. The cavity of a polished phone housing mold comes out bright and reflective, and a CMM scan confirms that the dimensions match the drawing to within a single micron.