45 Steel or P20 Steel | Cost, Hardness, Part Function

45 steel costs roughly RMB 5,000–7,000 per ton and reaches HRC 20–28 after quenching and tempering, making it suitable for mold bases and load-bearing parts.

P20 steel costs about RMB 9,000–12,000 per ton, comes pre-hardened to HRC 28–32, and offers good polishability, so it is widely used for cavities.

Material selection should be based on load requirements and surface-finish demands.

Cost

Raw Material Procurement

The listed price of P20 mold steel blanks from Fushun Special Steel stays fairly steady at around RMB 28.5 per kilogram. In-stock 45 steel round bars from Baosteel sell year-round at about RMB 6.2 per kilogram. Take a solid steel plate measuring 800 mm long, 600 mm wide, and 200 mm thick. Its volume is exactly 0.096 cubic meters.

Using the standard steel density of 7.85 g/cm³, the blank weighs 753.6 kg on the weighbridge. Buying that block in 45 steel costs RMB 4,672. Switching the material to P20 drives the invoiced amount up to RMB 21,477. The extra RMB 16,805 is essentially the price of the costly alloying elements added during steelmaking.

When P20 steel is melted, the mill adds about 1.7% chromium and 0.4% ferromolybdenum in a tightly controlled ratio. This year, spot molybdenum prices on the international market climbed as high as USD 45,000 per ton. Domestic high-carbon ferrochrome has been trading at around RMB 8,900 per base ton. By contrast, 45 steel contains only small amounts of manganese and silicon in addition to iron and carbon.

Those expensive alloying elements also require more sophisticated forging. Freshly cast P20 ingots leave the furnace at over 1,000°C, then go under a 2,500-ton hydraulic press for repeated forging from all directions. Coarse network carbides are broken up by force. After that, technicians scan the plate inch by inch with an ultrasonic probe coated in couplant.

Only after passing ultrasonic inspection to the E/e level under SEP 1921 can the plate be shipped. 45 steel follows a conventional converter steelmaking route. The molten steel is cast into long sections and rough-rolled into shape. The delivered black-skin surface typically carries a 2–3 mm decarburized and oxidized layer.

When buying steel plate, actual machining allowances must go beyond the net drawing size. For a plastic mold base measuring 400 × 400 mm, the blank is usually cut with an extra 5 mm allowance on each side. Once the blank arrives at the workshop, it is placed on a gantry milling machine to remove the surface scale layer with uneven hardness.

· Black-skin delivery is about RMB 0.5 cheaper per kilogram

· Milling to a bright surface adds about RMB 1.2 per kilogram in machining cost

· Vacuum degassing adds RMB 800 per ton

· Precision grinding can control surface dimensional tolerance within ±0.05 mm

Removing 10 mm of surface scale effectively means milling away expensive alloyed material. Bright P20 chips pile up on the floor and are swept into woven bags. Scrap yards pay only about RMB 1.8 per kilogram for them. That material loss is recorded as an early-stage procurement cost.

A 9.6-meter truck carrying 13 tons from Huangyan Mold City to Wuxi costs about RMB 2,200 in highway freight. For a full truckload of 45 steel plate, freight accounts for nearly 3% of the material value. Since P20 itself is much more expensive, transport costs amount to less than 0.6% per kilogram.

Large steel traders keep substantial inventory of 45 steel plates ranging from 20 mm to 300 mm in thickness. If you need only a 50 kg piece, they can cut it off with a small saw. But when a customer orders P20 plate thicker than 400 mm in a special size, the trader has to place an order with Fushun Special Steel, and the lead time is usually at least 35 days.

Some overseas customers specify imported ASSAB 718H from Sweden. The landed cost shown on the customs declaration can approach RMB 65 per kilogram.

· Import declarations require 13% VAT payment

· Cutting a sample for spectrometer testing takes 3–5 working days

· Receiving staff verify the embossed number on the original mill certificate with a magnifier

· The material is packed on wooden pallets, coated with rust preventive oil, and tightly wrapped in plastic film

Before domestic P20 leaves the mill, it is kept in an annealing furnace at 600°C for a full eight hours to relieve internal stress. After cooling, hardness is checked and held within HB 290–330. By contrast, 45 steel is often delivered in the hot-rolled condition with residual heat-treatment variation still present.

When 45 steel goes straight onto a CNC machine, localized hard spots sometimes cause sudden insert chipping, ruining a carbide insert that cost RMB 120. A 30-ton heat of P20 yields only about 65% usable product after the head and tail sections containing shrinkage cavities and gas defects are cut off.

45 steel is much more tolerant of small porosity and inclusions in the melt. One continuously cast billet can be cut into several sections and rolled into product. From one ton of molten steel, about 920 kg of finished round bar can be produced. On a manager’s desk lies a mold material order weighing 3.5 tons, waiting for signature.

Processing & Manufacturing

A worker hoists a 300 kg rough blank onto the table of a Fanuc-controlled 850 machining center. Once the green start button is pressed, the spindle drives a 50 mm face mill at 1,500 rpm into the steel plate. The machine sounds completely different when cutting 45 steel versus P20.

A Mitsubishi carbide insert costs RMB 35. For roughing 45 steel, two inserts are enough for a 12-hour day shift. With pre-hardened P20, insert wear doubles, and five worn inserts may be discarded in a single day.

A 20-liter drum of Great Wall cutting fluid costs RMB 200 and is diluted 1:20 with water. The long chips from 45 steel often wrap around the toolholder and have to be pulled off with a hook. P20 produces small, brittle chips about the size of fingernails, which wash directly into the chip conveyor.

After rough machining, 45 steel is usually trucked 10 km to a heat-treatment shop. It is heated to 840°C in a vacuum furnace and then quenched in oil, producing a cloud of black smoke. The heat-treatment shop charges by weight, typically RMB 2.5 per kilogram.

Once quenched, 45 steel deforms from thermal expansion and contraction. A 500 mm straightedge placed across the surface may show a gap large enough to admit a 0.8 mm feeler gauge. The plate then has to go back onto a large surface grinder for correction. At RMB 80 per hour, flattening that one plate can take four hours.

P20 skips the quenching step altogether. It comes from the mill already at about HRC 30, so after rough machining it goes straight to finish machining. By eliminating shipping, furnace queuing, and cooling time, mold production can be shortened by five full days.

Drilling cooling channels in a mold is labor-intensive. A 12 mm deep-hole gun drill costs RMB 450. In 45 steel, a 300 mm deep hole can be drilled at 80 mm/min. In P20, feed must be reduced to 50 mm/min.

For intricate latch features on plastic parts, standard cutters often cannot reach. In those cases, a fitter machines a copper electrode and uses EDM. With a discharge current of 20 amps and a 0.05 mm spark gap between the electrode and the steel, 45 steel erodes faster, while P20 erodes more slowly and leaves a finer surface texture.

For inserts with through-slots, wire EDM is used. A Sodick machine threads 0.2 mm brass wire and cuts P20 at around 40 mm²/min. A 5 kg spool of wire costs RMB 85, and one spool can be consumed by running the machine continuously for three days.

After machining, polishing begins. Because 45 steel is less pure internally, even after sanding with 800-grit paper, small pinholes may remain. P20 has a denser structure. It can be polished step by step up to 2000 grit, then finished with W3.5 diamond compound on a wool felt wheel.

Polishing labor is billed at RMB 60 per hour. To bring the cavity of a P20 mobile phone case mold to SPI A2 mirror finish, a polisher may spend 16 hours at the bench, costing RMB 960 in labor. A standard 45 steel turnover-box mold, by contrast, may receive only a rough four-hour grind with a grinding wheel.

All parts are then assembled and checked on a spotting press. Red lead paste mixed with oil is applied to the parting surface. The upper and lower mold halves are closed under 100 tons of hydraulic pressure and then separated again. With 45 steel, fit-up often leaves a 0.03 mm gap. P20, with its higher hardness and better dimensional stability, allows the fitter to stone the surface gradually and keep the gap within 0.01 mm.

Lifecycle

A 250-ton Haitian injection molding machine runs 24 hours a day. ABS pellets are heated to 220°C in the barrel until molten. The screw injects the material into the cavity at 120 bar pressure. With 15°C cooling water circulating through the mold, one part solidifies in 35 seconds.

A temporary mold base made from 45 steel cannot withstand repeated exposure to that temperature and pressure. When the shot counter reaches 58,000 cycles, a quality inspector checks the molded part with calipers and finds flash thicker than 0.15 mm along the edge. The entire box of parts is rejected.

The corners of the mold parting line have been washed away by the high-pressure plastic flow, leaving sharp, excessive flash on the edges of the molded parts.

The workshop supervisor orders production stopped. The four quick-connect cooling lines are removed, and an overhead crane lifts the 800 kg 45 steel mold into the repair room. Every hour of downtime on an idle molding machine costs about RMB 300 in depreciation, labor, and floor-space loss.

A mold technician repairs the damaged edge using TIG welding.

· One box of 2.4 mm ER50-6 welding wire costs RMB 45

· The welding heat softens the steel within a 30 mm surrounding zone

· Re-milling and hand polishing consume three full day shifts

· Three days of downtime delay the delivery of 25,000 plastic parts

Now replace that mold with a production mold made from P20. The cycle counter keeps climbing. Even at 350,000 shots, the white plastic housing still has a reflective surface. The chromium carbides in the material act like tightly packed pebbles, resisting erosion from the molten plastic.

A P20 mold typically needs only a 30-minute stop every 100,000 cycles for cavity cleaning with a dry cloth and a bottle of RMB 35 cleaner.

For low-volume, slow-moving spare parts, workshop owners often make do with 45 steel. One older water-dispenser tray component may have a total order of only 10,000 pieces. The machine runs for five days, finishes the order, and the 45 steel mold is then heavily greased and left in storage.

For a new robotic vacuum cleaner housing produced for a major brand, the first order may already be 500,000 pieces. With a tooling budget of RMB 120,000 approved, the buyer orders pre-hardened P20 plate exactly to drawing. At 8,000 housings shipped per day, the injection line runs for two months without a single mold seizure.

After 500,000 housings are delivered, the mold is removed and sent for full maintenance in a temperature-controlled area.

· A dial indicator shows only 0.02 mm wear on the four guide pillars

· The cavity is sprayed with WD-40 and wrapped in two layers of transparent plastic film

· The mold is moved into an automated storage system to await a repeat order of 300,000 units next year

· Spread across total output, the mold material cost is less than RMB 0.20 per plastic shell

The rusty 45 steel water-dispenser mold stored in the corner eventually reaches the end of its life. After sitting there for three years, it is sold as scrap. The scale shows 420 kg. At RMB 1.5 per kilogram, it brings back only RMB 630.

Hardness

45 Steel

Experienced machinists can tell how 45 steel will cut almost by feel. Fresh round bar from the mill is not especially hard, usually around 170–197 HBW. Clamp a 50 mm diameter bar in the lathe chuck, fit an ordinary coated carbide insert, and begin peeling the surface.

Set the spindle to 800 rpm, and the chips come off like bright silver noodles. The tool can take a full 3 mm depth of cut, while the spindle load meter holds steady at 40%. Friction at the cutting edge reaches about 500°C, yet the tool can still cut dry without coolant.

Turning a one-meter shaft consumes about half an insert. At a feed of 0.2 mm per revolution, the freshly turned surface feels somewhat rough to the touch. A roughness tester typically reads Ra 3.2–6.3 μm. That is standard rough machining.

If you want to harden the material, the bar has to go to heat treatment. The induction heater hums, and the operator holds the furnace at 840°C. The steel gradually turns bright cherry red. After soaking for 30 minutes, it is pulled out and immediately quenched.

The quench tank is filled with room-temperature water. A red-hot part at 600°C can cool at roughly 600°C per second. To intensify the effect, workers often add coarse salt at around 10% concentration.

· The salt breaks up the vapor blanket on the surface

· Cooling speed can rise to 1,200°C per second

· Surface hardness can jump to 55 HRC

But higher hardness also brings brittleness. A part just pulled from salt water can chip at the corner if dropped on concrete. The operator then tempers it in a 200°C oven for two hours. After that, hardness drops back to 45–50 HRC.

Engine connecting rods require a different balance: hard enough, but tough inside. That means tempering at a much higher 500°C. After treatment, Rockwell testing is no longer ideal, so Brinell testing is used instead. The reading typically lands at 220–280 HBW. On a tensile tester, the part can withstand over 355 MPa without fracture.

Still, 45 steel has a physical limit that is hard to overcome. Take a solid bar 100 mm in diameter, heat it, quench it hard, let it cool, then cut it cleanly through the center and test hardness from surface to core.

· 2 mm below the surface: 50 HRC

· 10 mm below the surface: 35 HRC

· At the centerline: 18 HRC

Water quenching does not fully harden the core. Under a microscope, the center still shows a relatively soft original microstructure. In heavy machinery under high torque, the outside may remain intact while the core twists internally. For heavily loaded parts thicker than 30 mm, buyers usually switch to 40Cr or the more expensive 42CrMo.

At the steel market, standard bright 45 steel bar sells for around RMB 4,500–4,800 per ton—less than RMB 5 per kilogram. Compared with specialty mold steels that cost tens of yuan per kilogram, it is extremely inexpensive.

But the low price comes with additional work. Outsourced water quenching plus tempering usually costs RMB 2.5–3.5 per kilogram. Slender, long parts often warp badly during treatment, and straightening can add another 30% to 50% in cost.

Veteran fitters can often identify 45 steel by spark testing alone. Press a scrap piece against a high-speed grinder, and the sparks come out long and fine, branching like a tree—exactly what you would expect from a steel with 0.45% carbon.

On rainy days, when humidity climbs above 70%, uncoated 45 steel on a shelf can develop a yellow-brown rust film in just three days. Measured with calipers, that surface rust can consume 0.05 mm of dimensional tolerance.

If a drawing calls for a hardened surface layer 1.5 mm deep, an induction coil scans the outer diameter at 15 mm per second while a water spray ring follows behind. The red-hot surface, just at 850°C, is quenched down to 30°C in two seconds. Hardness measured at the tooth flank stabilizes at 52 HRC.

P20 Steel

A technician uses a 10-ton crane to lift a massive square block of P20, one meter by one meter and half a meter thick. On the weighbridge, it comes to a full four tons.

The delivery note states a hardness of 28–32 HRC. A quality inspector checks five random points on the surface with a portable Leeb hardness tester, and the results vary only from 29.5 to 30.2.

The steelmaker adds 1.4%–2.0% chromium and about 0.3% molybdenum to the melt. Large steel mills place these multi-ton blocks into a huge car-bottom furnace and heat the entire mass to 860°C.

After soaking, the steel is oil-cooled slowly, then tempered continuously for 48 hours in a large furnace at 600°C. Even when the four-ton block is cut through the middle on a giant band saw, hardness at the center still reads 29 HRC.

The hardness difference between surface and core is less than 1 HRC. In the mold shop, that means the block can go straight onto a three-axis gantry mill. If the drawing calls for 600.00 mm by 600.00 mm, the machining program can cut directly to size without leaving heat-treatment allowance.

With a 63 mm four-insert face mill at 1,200 rpm, contact sparks are minimal. The chips are dark blue, short, and easy to break. They do not form long spirals.

The machine feeds at 800 mm/min. The carbide inserts remove 3 mm of material in one pass, and spindle load stabilizes at 65%. Because the material is already pre-hardened, tool wear is faster, and a single eight-hour shift may consume two full sets of inserts.

Complex 3D mold surfaces rely on CAM programming. An 8 mm ball end mill steps down in 0.1 mm increments. After 48 hours of continuous cutting, the recessed cavity for a television back cover is formed, with dimensional tolerance held within ±0.015 mm.

For larger molds such as automotive bumpers, buyers often specify an upgraded version of P20. Adding 0.8%–1.2% nickel creates P20+Ni, with hardness increased to 33–38 HRC.

Every morning, purchasing staff check the latest steel market price list.

Polishers who work with molds know P20’s behavior well. They begin with 400-grit wet sandpaper to remove machining marks, then move to 800 and 1200 grit with kerosene for cross-polishing.

Next comes W3.5 diamond compound and a high-speed wool wheel. In just two hours, a dull gray cavity can be brought to a mirror-like surface. Surface roughness reaches Ra 0.25 μm.

A mold weighing 1.5 tons is then mounted on an 800-ton injection molding machine. ABS melt at 260°C is injected into the cavity at 120 MPa. Cooling water at 15°C flows through the channels, freezing the plastic within 20 seconds.

The machine runs 24 hours a day in three shifts. Even after 300,000 television housings, caliper checks on the latch features show dimensional variation of no more than 0.03 mm compared with the first shot.

The surface hardness stands up to repeated erosion from high-temperature plastic. If a corner is accidentally damaged during mold repair, the welder uses dedicated P20 TIG filler wire. A 2 mm-wide damaged edge can be repaired at 110 amps.

After cooling, a file cuts cleanly through the repaired zone. The microstructure shows no obvious embrittlement, and hardness still stays around 30 HRC. After repolishing, the weld blends visually into the surrounding base metal with no visible color difference.

Part Function

High-Quality Carbon Structural Steel

In molten 45 steel, sulfur is strictly limited below 0.035%, and phosphorus must also stay under 0.035%. Ordinary reinforcing steel for construction can tolerate 0.050% impurity content. That seemingly small 0.015% difference has a direct effect on tool wear during machining.

Too much carbon makes the steel brittle; too little makes it too soft. That is why the carbon range must stay tightly between 0.42% and 0.50%. Below 0.42%, even after quenching, hardness may not reach HRC 40. Above 0.50%, the material becomes prone to fracture under heavy loads at spindle speeds of 3,000 rpm.

In heat treatment, the operator places the round bar into an electric furnace at 840°C. The thicker the bar, the longer the soak. A common shop rule is one additional minute of holding time for every additional millimeter of thickness. Once the furnace door opens, the glowing steel must be plunged into water within 3–5 seconds.

The quench water must not exceed 30°C. Surface temperature needs to drop at about 600°C per second. The internal structure rapidly transforms into a hard martensitic state.

· If water temperature rises to 50°C, soft spots will appear on the surface

· If immersion is delayed by 8 seconds, the grains coarsen

· If cooling falls below 200°C per second, hardness will fail to meet spec

· Sharp-cornered parts are prone to hairline cracks during quenching

· If the part is pulled out while still at 150°C internally, stored stress may cause cracking

The part is then tempered at 500–600°C for two full hours. Once the brittleness is relieved, yield strength stabilizes at 355 MPa and tensile strength comes back down to the acceptable level of 600 MPa.

In the machine shop, hardness readings typically fall in the comfortable HB 200–240 range. A lathe insert cuts it cleanly, producing blue-violet curled chips. When the chips come off continuously in long strips, machinists know the hardness-toughness balance is right.

Lead screws and other wear-prone parts often need a hardened surface shell. A copper induction coil is placed around the shaft and energized at 200 kHz. In about three seconds, the outer 1.5 mm layer rises to 850°C.

A spray ring immediately follows with water at 0.15 MPa. The outer surface reaches HRC 50, while the core remains around HB 220 and retains toughness.

· Induction frequency: 100–500 kHz

· Hardened layer depth: 0.5–2.0 mm

· Surface hardness: HRC 45–55

· Spray pressure: 0.1–0.2 MPa

· Travel speed: 3–5 mm/s

Grinding wheels then finish the surface at 35 m/s. Dimensions can be held within 0.005 mm, and surface roughness can be maintained at Ra 0.8 μm.

A gear blank for a gearbox may measure 200 mm in diameter and 50 mm thick. A hobbing machine cuts the teeth at a cutting speed of 40 m/min. The drawing typically leaves 0.2 mm stock for later finishing.

At the steel market, 45 steel looks much like ordinary mild steel. But an experienced fitter can identify it instantly by spark testing: 45 steel gives forked, fireworks-like sparks, while low-carbon steel with only 0.20% carbon gives almost straight sparks with little branching.

A steel dealer’s warehouse may stock bars ranging from 10 mm to 300 mm in diameter. Standard bars are 6 meters long and still carry the black oxide skin from hot rolling. Weight is always calculated using the standard density of 7.85 g/cm³.

· Cold-drawn bright bar diameters: 5–80 mm

· Hot-rolled black bar diameters: 50–250 mm

· Forged bar diameters: 200–800 mm

· Plate thicknesses: 10–150 mm

Before welding, workers preheat the area with an oxy-acetylene torch to 150–250°C. They then build up the weld with E7015 low-hydrogen electrodes.

After welding, the part cannot simply be left to cool on its own. It is wrapped tightly in insulating blankets, and the cooling rate must stay below 10°C per minute. Once fully cooled, it is placed in a furnace at 600–650°C for three hours to relieve residual stress.

Traditional claw hammers used by carpenters are also made from steel of the same composition. The steel is heated to 900°C and forged into shape. The striking face is water-quenched to about HRC 45, while the claw section needs only HRC 35; if it were harder, it would chip during nail removal.

At an agricultural machinery repair station, a mechanic rebuilding a 60 hp diesel engine may cut a connecting rod cap from a 100 mm thick steel plate using wire EDM. With a cutting rate of 50 mm² per hour, the finished part can run on the crankshaft for 8,000 hours without deforming.

At construction sites, the anchor bolts under tower cranes are often large M36 threaded rods made from this material. Their 60-degree thread profiles hold massive steel structures in place, even under tightening torque of 2,000 N·m, without thread stripping.

In a hardware factory, the solid shaft driving the 500 kg flywheel on a 200-ton mechanical press is also made from this steel. Even under 60 strokes per minute, the shaft withstands repeated cyclic loading without failure.

In East China’s major steel markets, cold-drawn round bar typically sells for RMB 4,500–4,800 per ton. Large machinery plants may consume over 300 tons of solid round bar every month. Freight is commonly billed at RMB 0.5 per ton-kilometer.

The chips under the lathe are not waste. Long curled scrap sells for about RMB 2,200 per ton, while chunkier offcuts may bring RMB 2,600 per ton. Briquetting machines compress iron chips into 600 mm blocks for remelting.

Low-Carbon Alloy Tool Steel

Inside the steelmaker’s furnace, molten 3Cr2Mo is alloyed with 1.4%–2.0% chromium and 0.3%–0.55% molybdenum. Chromium improves corrosion resistance and polishability, while molybdenum helps maintain uniform hardness even in blocks as thick as 500 mm.

Once the melt reaches 1,600°C, a vacuum hood is lowered and the steel is degassed. Hydrogen is reduced to below 2 ppm and oxygen to below 15 ppm. Skipping this step to save money would leave pinhole porosity after cooling.

The red-hot ingot is then forged repeatedly under a 5,000-ton hydraulic press to break down coarse grains. Before shipment, the steel mill performs a full pre-hardening treatment, locking hardness into the HRC 28–32 range so the buyer can machine it directly.

Ordinary steel must be quenched after machining, and the hot-cold cycle can distort dimensions by 0.2 mm. Pre-hardened material eliminates that step. A CNC machine can mill directly to drawing size and hold dimensional error to 0.01 mm.

Before shipment, inspectors scan the surface with an ultrasonic flaw detector. If the signal peaks on the screen, internal cracks may be present deep inside an 800 mm thick block.

At 8,000 rpm spindle speed and 1,500 mm/min feed, the chips come off not as long strings but as small broken granules, showing how evenly the alloy is distributed.

Cooling channels are drilled through the mold using a long gun drill. With an 8 mm drill, holes 400 mm deep can be made straight and clean because the pre-hardened material does not gall the tool. Once in service, high-pressure water can remove 200 kcal of injection heat per minute.

Where cutters cannot reach, EDM is used. A copper electrode is separated from the steel by a 0.05 mm kerosene gap. At 30 amps and 20,000 discharges per second, it burns a cavity that precisely matches the electrode shape.

For narrow slots only a few millimeters wide, wire EDM uses 0.18 mm molybdenum wire cutting through water. The process is slow—only about 40 mm² per hour—but it leaves a sidewall as smooth as a fine abrasive finish.

Molds for television housings need a highly reflective finish. The polisher starts at 400 grit and works up to 1500 grit, then applies 3 μm diamond compound. With a wool wheel running at 15 m/s, the surface can be polished to Ra 0.05.

Artificial leather textures on automotive dashboards are chemically etched. A patterned film is applied to the mold surface, then ferric chloride solution is brushed on. After 45 minutes, the exposed areas are etched to a depth of 0.08 mm, creating a realistic grain texture.

Two finished mold halves are mounted on an 800-ton injection molding machine. Plastic melt at 260°C is injected at 120 MPa. The mold surface, backed by a base hardness of HRC 30, withstands the enormous clamping and injection forces without denting.

After 15 seconds, the plastic solidifies and the mold opens to eject the part. The parting line closes and opens millions of times. Over a 300,000-shot service life, the edges do not collapse, and the molded parts remain free of sharp flash.

When a mold corner is damaged after long use, repair is delicate work. The TIG welder is set to just 50 amps, and 0.8 mm filler wire of matching material is applied carefully. The repaired area may rise to HRC 45 after welding, so it must be tempered back at 400°C before it can be polished again.

Functional Applications

On wooden pallets in the workshop lie freshly turned 45 steel drive shafts, each weighing 12 kg. They are destined for rotary tiller gearboxes on agricultural tractors. The splined shaft ends engage the gears and withstand the severe torque generated during daily tillage over 40 mu of dry land.

Tractor chassis have virtually no vibration damping in muddy fields. The machine’s resistance to vibration depends heavily on the tempered sorbite structure inside the material, which must endure around 800 severe vibration cycles per minute.

The same batch of round bar may also be cut into 50 mm sections, drilled on a press, and made into non-standard flanges. In one chemical plant application, these flanges are installed on 200 mm wastewater pipelines. Eight M16 bolts clamp them in place tightly enough that corrosive wastewater at 0.6 MPa does not leak.

The central forcing screw in a three-jaw puller used by auto repair shops is also machined from this material. When the mechanic fits an extension bar and starts turning, the screw tip presses against the center of a rusted brake disc.

· The threaded tip pushes against the center hub

· The three jaws lock tightly behind the edge

· The screw generates nearly 3 tons of pushing force

· The seized bearing finally breaks loose with a dull pop

A machinist may place a 300 mm square blank on a milling machine, face it flat, and drill eight rows of threaded holes in both directions. Under a heavy flood of coolant, the base plate stays dimensionally stable and can hold tools at the coordinate origin within a 0.02 mm error zone.

In the adjacent temperature-controlled shop, P20 steel is used very differently. A pre-hardened block 1.2 m long and 0.8 m wide lies on a five-axis machining center. Its future role is to form the front bumper of a BYD new-energy vehicle worth RMB 200,000.

A mold block weighing 1.5 tons cannot contain even the slightest crack. Inside the injection molding machine, 280°C molten polypropylene fills the cavity like lava in just five seconds.

An automotive bumper requires a mirror-smooth outer surface and cannot tolerate shrinkage marks deeper than 0.1 mm. At mold closure, four 150 mm diameter tie bars stretch to apply 1,200 tons of clamping force. If the steel were any softer, the cavity surface would be crushed.

Small molds for cosmetic bottle caps rely entirely on P20’s polishing performance. A lipstick cap that will later be electroplated in bright silver must start with an absolutely mirror-finished plastic surface.

· The polisher applies 1 μm diamond paste under a microscope

· Bamboo sticks are used to work the corners for 40 full hours

· Mold surface reflectivity is pushed to 98%

· The molded plastic cap is reflective enough to be used like a small mirror

A refrigerator drawer mold in a Haier appliance plant may remain in service for three full years. Every opening and closing cycle causes the slide components to move along the guide rails. After 150,000 shots, the fit clearance has increased by only 0.03 mm, and the transparent drawer edges are still clean and crisp.

Toy manufacturers producing interchangeable building blocks demand extremely tight dimensional control. The mating accuracy between two blocks depends on cavity precision of 0.005 mm. Because P20 releases very little residual stress during machining, even after half the internal material is machined away, the external contour remains straight with no warpage.

On construction sites, if the shaft in a mortar mixer wears out, the mechanic can simply buy a standard 45 steel replacement shaft from a hardware store. A 600 mm long spare part may cost only RMB 85, yet it can continue mixing tons of stone and cement.

Replacing that shaft with pre-hardened alloy steel would add more than RMB 300 in material cost alone, and even welding on a small tab would require preheating. That would be using an overly expensive solution for a basic repair.

Some inexperienced shops try to save RMB 2,000 by machining a mobile phone case mold from ordinary round steel. The moment the milling is done and internal stress is released, the once-flat surface may warp by 0.5 mm. If they then attempt to harden it by quenching, the long mold comes out bent like a banana.

When that defective mold goes onto the injection line, every plastic shell comes out with sharp flash along the edge. Workers trim it with utility knives, leaving white stress marks on the plastic. In the end, the whole batch of 5,000 housings is shredded into scrap plastic.