How to Buy P20 Steel | Hardness, Size Range, Machining Support

When purchasing P20 steel, confirm a pre-hardened hardness of HRC 28–32, check whether the size range such as a thickness of 100–400 mm provides sufficient machining allowance, require the supplier to provide an ultrasonic inspection report, leave 1–2 mm allowance for rough machining, and carry out stress-relief annealing when necessary to ensure stable downstream machining.

Hardness

Pre-Hardening and Parameters

Steel mills heat P20 to 860–880°C. For every additional 25 mm of plate thickness, the material remains in the furnace for one extra hour. High-pressure nitrogen or dedicated quenching oil is then used for rapid cooling to force a transformation in the steel’s internal microstructure.

After cooling, the plate is transferred to a vacuum furnace at 550–650°C and soaked for 4–6 hours. The surface hardness stabilizes at 280–320 HB, while the internal grain structure becomes highly uniform.

The upper limit of 28–32 HRC directly constrains CNC machining speed. With a 12 mm carbide milling cutter, the cutting speed can only be set at 80–120 m/min. Push the spindle speed too high, and the insert may chip instantly.

· Rough machining feed per tooth: 0.15–0.30 mm

· Finishing depth of cut: 0.05–0.15 mm

· High-speed steel drill cutting speed: 15–22 m/min

· Tapping speed: strictly limited to 5–8 m/min

When deep-hole drilling hits a local hardness point of 32 HRC, wear on an M35 cobalt drill accelerates dramatically. A drill that would normally complete 200 holes may become fully dull before reaching 80. Consumable tooling costs in the workshop can rise sharply as a result.

Ultrasonic inspection is carried out strictly in accordance with German SEP 1921 Class D/d. No internal pores or microcracks larger than 2 mm are acceptable in thick steel billets. A clean internal structure ensures that hardness does not fluctuate abruptly during machining.

If a soft zone below 27 HRC is present, it becomes a serious problem during high-gloss polishing. Even with 3000-grit diamond compound, the softer areas are over-polished, leaving fine pinholes across the surface. Surface roughness may stall at Ra 0.05 μm and go no lower.

· Leeb hardness tester: must use a standard D-type impact device

· Surface finish before testing: Ra below 2.0 μm

· Spacing between adjacent test points: 3 mm

· Minimum distance from the edge: 5 mm

For extra-thick P20 plates over 500 mm, both an ultrasonic thickness gauge and a portable hardness tester should be used. When the material is sectioned and the center is measured, it becomes clear that thick plates cool extremely slowly. At the core, the cooling rate may be less than 15°C per minute.

With 0.38% carbon and 1.50% manganese, such slow cooling encourages ferrite network formation along grain boundaries. In very thick plates, the actual core hardness may fall to 24–26 HRC. Dies made from such soft-centered material are much more prone to cavity collapse and deformation.

By adding 1.0% electrolytic nickel during steelmaking, the cooling transformation curve can be altered significantly. Even when the core of a large section cools as slowly as 8°C per minute, the structure can still transform into strong lower bainite.

· Carbon equivalent: tightly controlled at 0.75–0.85

· Non-metallic inclusions: Type A fine sulfides not above 2.0

· Austenite grain size: ASTM 5–8

· Hardness variation across the same section: no more than 20 HB

In the forging shop, a 6000-ton hydraulic press repeatedly works the steel, maintaining a forging ratio above 4:1. Dendritic structures formed during solidification are crushed under massive pressure. After heat treatment, hardness fluctuation across the section can be held within 1.5 HRC.

When an injection mold closes under pressure, the mold base may face an expansion load of 150 MPa. A hardness of 30 HRC corresponds to a tensile strength of about 980–1080 MPa. After deformation under load, the material can elastically recover, withstand one million molding cycles, and keep dimensional error below 0.01 mm.

For chemical texturing, surface hardness uniformity is especially critical. Under the VDI 3400 standard, even a small difference between 28 HRC and 32 HRC can create a 15% difference in etching rate.

Uniformity and Composition

During steelmaking, furnace temperature rises to 1550°C and alloying elements are added to the molten steel. Chromium is kept within a narrow range of 1.80%–2.00%. This allows a 200 mm thick plate to harden from the surface all the way to the core during quenching. Without enough chromium, the center becomes soft after sectioning.

Molybdenum is added at only 0.15%–0.25%, but it accounts for a major share of alloy cost. Even at that level, it greatly reduces the risk of temper brittleness. When quenched plates are reheated to 600°C, molybdenum helps preserve internal toughness.

Silicon is maintained at 0.20%–0.40% for deoxidation. Too much silicon increases blocky non-metallic inclusions, which can chip CNC drills on contact. Manganese is raised to 1.50% to support deoxidation and also increases tensile strength by about 80 MPa.

Sulfur and phosphorus are natural impurities in molten steel, and both must be kept below 0.03%. Above that level, molds may crack at the corners after 500,000 cycles. Higher-grade P20 can be refined through ESR, reducing sulfide inclusions to as low as 0.005%.

A 20-ton steel ingot cools naturally in a sand pit. The outer shell solidifies within five hours, while the center remains liquid and turbulent at 1200°C for as long as 30 hours. During this slow cooling process, carbon migrates toward the center.

This carbon enrichment causes significant chemical segregation. When a 600 mm ingot is cut open and tested, the outer layer may read 31 HRC, while the center drops to 26 HRC.

To reduce cross-sectional hardness drop, steelmakers add about 1.0% nickel in grade 1.2738. Nickel stabilizes the transformation of the slow-cooled center into harder lower bainite. Even in an 800 mm section, core hardness can still hold at 29 HRC.

On spectrometric analysis, the content of 16 elements may appear on screen instantly. If carbon falls from the target 0.40% to 0.33%, the finished plate will never reach 28 HRC at the surface.

An 8000-ton hydraulic press repeatedly forges the red-hot ingot, alternating between drawing and upsetting over six cycles. A total forging ratio of 6:1 helps break up clustered carbides and segregated alloy areas.

Workers etch the cut section with 10% nital and inspect the macrostructure by eye. Dendritic segregation must be blurred by forging. If pores over 1.5 mm remain, the ultrasonic screen will show dense, irregular peaks.

At 500× magnification, insufficient forging reveals banded black-and-white microstructures. If banding exceeds Level 3, the cutting tool may vibrate microscopically during CNC machining.

If the purchased material has uneven hardness, polishing becomes much more difficult. With 5000-grit abrasive, a 27 HRC zone may wear 20% faster than a neighboring 31 HRC zone. Under light, the mold surface will show visible ripple-like depressions.

For automotive dashboard molds, leather-grain texture is created by acid etching. Workers brush a mixed acid of nitric acid and ferric chloride onto the surface. In harder zones, the etch depth may be only 0.03 mm.

In softer zones, the acid may penetrate to 0.05 mm almost immediately. Once the chemical is removed and plastic parts are molded, the surface shows uneven color and texture. The entire mold may need to be scrapped and reworked by removing the texture with EDM to a depth of 0.5 mm.

For hardness mapping, indentations are taken every 10 mm. On a plate 1 meter long and 500 mm wide, about 100 test points may be required. All readings must stay within 285–315 HB.

Nitriding Technology

When 30% short glass fiber is blended into molten ABS, the material becomes highly abrasive. The base hardness of untreated P20 at about 30 HRC is not enough to resist that wear.

If a bare mold is used without nitriding, the gate area may wear by 0.05 mm after only 50,000 molding cycles. Plastic parts for household appliance housings may then develop sharp flash along the edges, requiring manual trimming.

The mold is suspended into a deep vacuum pit furnace and heated to 510–530°C. Temperature must never exceed 550°C, or the tempered sorbite structure inside P20 will break down and core hardness may drop below 25 HRC.

Liquid ammonia of 99.9% purity is fed into the furnace through stainless steel piping. At high temperature, it decomposes into active nitrogen atoms and hydrogen. The ammonia dissociation rate is strictly controlled at 30%–45%.

· Ammonia flow rate: 2.5 m³/h

· Furnace pressure: 200–300 Pa positive pressure

· Exhaust gas check: hydrogen content sampled every 30 minutes

· Holding time: 15–40 hours, depending on section thickness

Active nitrogen diffuses into the surface lattice of the steel. At 520°C for 24 hours, nitriding depth can reach 0.15 mm. Extending the treatment to 48 hours usually increases depth to no more than 0.25 mm.

Using a Vickers hardness tester, the treated surface may rise from 300 HV before nitriding to 650 HV, equivalent to roughly 58 HRC.

Under the metallographic microscope, the outermost layer appears as a 10–20 μm bright white band. This is the so-called “white layer,” or compound layer, which is brittle like glass and may spall under impact.

Polishers must remove this white layer completely with an 800-grit oil stone, retaining the 0.12 mm diffusion layer beneath it. That diffusion layer contains fine alloy nitrides, combining hardness around 60 HRC with some residual toughness.

A different method is ion nitriding in a vacuum furnace. After evacuating below 10 Pa, ammonia is introduced and the mold is energized with 500 V DC. The electric field ionizes the nitrogen into a reddish-purple plasma glow that surrounds the mold.

· Heating rate: 150°C/h

· Process cycle: 30%–50% shorter than conventional gas nitriding

· Distortion: hole-position deviation below 0.005 mm

· Case uniformity: no excessive brittle buildup at sharp corners

Workers apply two thick coats of anti-nitriding paint containing 80% copper inside threaded holes. This blocks nitrogen penetration, keeping the hole hardness at around 30 HRC.

A witness sample processed alongside the mold is sectioned and tested. After polishing and etching, a microhardness tester records hardness every 0.02 mm from the surface inward to generate a hardness profile.

At 0.05 mm below the surface, hardness may read 620 HV. At 0.10 mm, it may fall to 510 HV. Beyond 0.15 mm, hardness drops back to the base level of around 310 HV. That 0.15 mm case is what protects against glass-fiber wear.

When molding PA66 with 30% glass fiber, a gate that would normally wear out after 50,000 cycles may still show less than 0.01 mm wear after 250,000 cycles.

Size Range

Round Bar

Commercial P20 round bar ranges from 16 mm up to 1000 mm in diameter. For smaller sizes below 130 mm, steelmakers often use electric arc furnaces followed by two stages of refining and degassing. Carbon is tightly controlled at 0.35%–0.38%.

For diameters above 130 mm, forging with hydraulic presses of over 10,000 tons is required. A 3-ton block heated to 1200°C is repeatedly worked to achieve a forging ratio above 4:1, eliminating internal voids.

Before material enters stock, grain size must be verified at Level 6–8 under the microscope. Sulfides and oxides must stay below Level 1.5. For mirror-polished parts finished to 12,000 grit, ESR material is preferred for its cleanliness.

Before purchase, ultrasonic inspection should be used to check for internal cracks. Typical requirements include:

· Tested in accordance with SEP 1921-84

· For sizes below 300 mm: Class C/c

· For 300–600 mm: Class D/d

· Ultrasonic frequency: 2 MHz or 4 MHz

· Blind zone below 15 mm

A portable hardness tester is typically used to measure five random points on the bar end, and results generally fall within 28–32 HRC. If 0.8%–1.2% nickel is added, the upgraded version can reach 35 HRC.

On a 200 mm bar, the outer 1.5–2 mm is often a thermally damaged skin. When turning, at least 3 mm per side should be removed to expose sound, hardness-uniform material.

The straightness tolerance from the steel mill is typically up to 3 mm per meter. On a 5.8 m bar of 80 mm diameter, the height difference between both ends may reach 17.4 mm. Hydraulic straightening is often required before machining.

In the workshop, speed and feed must match hardness:

· Rough turning with carbide: 80–120 m/min

· Feed: 0.2–0.4 mm/rev

· Depth of cut: 2–4 mm

· With HSS tools: 20–25 m/min

· Drill point angle: 118°

Cutting off a 400 mm round bar is time-consuming. A band saw may need about 45 minutes. Coolant should be mixed at 1:10 concentrate to water. The cut face may show unevenness of about 3 mm.

Guide pillars in mold shops often suffer from uneven cooling. With 1.4%–2.0% chromium and 0.3%–0.55% molybdenum, P20 shows a thermal conductivity of about 29.3 at 20°C, changing only slightly to around 29.5 at 200°C.

For cylindrical moving components exposed to repeated friction, nitriding is often added after machining. Under vacuum, ammonia is introduced and the part is held at 510°C for 48 hours. The outer 0.2–0.3 mm can then exceed 650 HV.

Flat Bar and Plate

P20 flat bar and plate cover a wide size range, from 16 mm plate to massive forged sections up to 1000 mm thick. Width commonly starts at 200 mm and can reach 2200 mm. Large automotive bumper or TV housing molds often require these large plates.

Most plates below 130 mm thick are produced by hot rolling. Above that thickness, large hydraulic presses are used for six-side forging. A forged block measuring 3000 × 1500 × 800 mm can weigh nearly 28 tons.

Large plates often show a hardness difference between the surface and the center. A 500 mm P20 plate may measure 30 HRC on the surface but only 26 HRC at the center because of slower cooling during quenching.

When purchasing rough plates with black oxide scale, more machining allowance must be reserved. At least 3–5 mm per side should be left on thickness. Saw-cut width and length are also uneven, so an extra 5–10 mm should be reserved around the perimeter.

Stock-size tolerance tables can be used to verify available dimensions:

Suppliers often machine all six faces on a large double-head milling machine. Surface roughness can reach Ra 3.2, and on a 1000 mm plate, the diagonal difference is kept below 0.5 mm.

Flatness is also tightly controlled, with height variation limited to 0.1 mm per meter. Sawing thick plates is a slow process. A long bi-metal band saw is typically run at 20 m/min.

Cutting a plate 400 mm thick and 800 mm wide may take about 3.5 hours. Large sections are prone to stress release. After a cut is made, the opening at the ends may spring outward by 5–8 mm.

To prevent deformation during later machining, high-precision mold sections are often stress-relief annealed at 550°C for several hours after cutting. Ultrasonic testing of plate is more difficult than for round bar; the probe must scan a grid pattern across the broad surface. Under GB/T 2970-2016, plates over 200 mm thick require at least Class II inspection.

Ultrasonic signals can detect white spots or shrinkage cavities over 5 mm in the middle of the plate. Once on the CNC machining center, spindle speed is adjusted according to hardness. For rough face milling, spindle speed is typically 400–600 rpm, with a 2 mm depth of cut.

After profile milling, waterline holes are drilled using tungsten carbide drills at a feed of 0.15 mm/rev. Continuous curled chips indicate that hardness and toughness are well balanced. Stock plate length usually ranges from 2000 mm to 5800 mm.

For local truck delivery, a 13-meter flatbed can carry about 30 tons. Oversized or overweight modules require special low-bed transport and night permits. During unloading inspection, a portable Leeb hardness tester is commonly used.

Typical readings fall within 450–510 HL, which converts closely to the standard range of 28–32 HRC.

Machining Allowance

Buying P20 rough stock with dimensions too close to the finished size is risky. Black-skin steel plate exposed to air often develops a decarburized surface layer 0.5–1.2 mm thick. In rough machining, the first cut should remove at least 2 mm to reach sound material.

In the decarburized layer, carbon has been burned out and hardness may not even reach HRC 20. If used as a working surface, the mold may collapse after only a few thousand molding cycles.

For small blocks, the standard practice is to leave 3–5 mm per side in length, width, and height. A plate measuring 300 × 400 mm may show a saw-cut height difference of 1.5–2.5 mm, so ordering 310 × 410 mm is safer.

For plates over 1000 mm, leaving only 5 mm per side is often insufficient. Large plates contain significant internal stress, and after cutting, warpage may produce a 4–6 mm height difference. Experienced buyers may order a 1500 mm plate with a minimum size requirement of 1515 mm.

· Below 500 mm in length/width: 3–5 mm per side

· 500–1000 mm span: 5–8 mm per side

· Above 1000 mm: 8–15 mm per side

If the plate is cut by oxy-fuel flame, allowance should be doubled at the edge. A 200 mm thick plate cut with a 2000°C flame develops a heat-affected zone extending 15–20 mm inward.

The hardened edge band can exceed HRC 45. If that layer is not removed before milling, a carbide cutter running at 800 rpm may destroy a full set of inserts in just two minutes.

Allowance must also be reserved for deep-hole drilling. A 10 mm gun drill boring 500 mm deep can easily drift inside the steel. Exit-point deviation may reach 2–3 mm. Without enough surrounding stock, the drill can break through the side wall.

For fully machined six-side flat stock, the approach is different. Since the supplier has already face-milled the material to around Ra 3.2, only a small finish allowance is needed:

· Face milling allowance: 0.2–0.5 mm per side

· Surface grinding allowance: 0.05–0.15 mm per side

· EDM allowance: 0.02–0.08 mm per side

After CNC roughing with a 20 mm face mill, the cavity surface may still retain about 0.5 mm of stepped stock per side.

After rough machining, the mold should sit on the shop floor for 2–3 days to allow residual stress to relax. Once a large volume is removed, the base may rise by about 0.15 mm.

When a 12 mm end mill cuts down 80 mm deep, deflection causes the tool to lean slightly under load. The bottom of the wall may end up 0.05–0.1 mm wider than the top.

For vertical walls, leaving 0.15 mm per side and then taking one final finishing pass with a new sharp cutter at 500 rpm helps maintain full dimensional consistency from top to bottom.

For precision components that will be nitrided, allowance must be controlled to the micron level. After 48 hours in a nitriding furnace at 510°C, the material may expand by about 0.02%. A guide pillar of 50 mm diameter may grow by nearly 0.01 mm.

This tiny expansion has to be accounted for before heat treatment. On the cylindrical grinder, a 60-grit wheel runs at 1440 rpm, and the final allowance left for polishing is kept within 0.005 mm.

When using pre-hardened P20 for thin-wall parts such as a 15 mm electronic panel mold, problems can arise quickly. If a thin slice is sawn from a 100 mm plate, the center may sink by 2 mm immediately after cutting. For thin parts, at least 4 mm per side should be left, and both faces should then be ground alternately to flatten the part.

Machining Support

Rough Machining and Allowance Control

When cutting large black-skin P20 round stock, a bi-metal band saw usually runs at 25–35 m/min. Downforce is kept at 30–40 kg. For material around 400 mm thick, a variable-pitch 2/3 or 3/4 tooth pattern is used to reduce vibration. Above 500 mm, a carbide band saw is preferred, and speed can be raised to 45 m/min.

The cut surface typically varies by 3–5 mm in flatness. When cutting to size, workers usually leave 5 mm per side. For plates over 500 mm thick, where blade wandering is more likely, 8 mm per side is safer.

· Blade tension: 250–300 N/mm²

· Coolant concentration: 8%–10%

· Distance from guide block to steel surface: 20–30 mm

· M42 saw blade life: replace after cutting 2 m²

· Entry angle adjustment: 1.5°–2°

Below the as-delivered surface of P20 lies a decarburized layer about 1.5–3 mm thick, often with uneven hardness and fine defects. Workers fit a 160 mm or 200 mm wide face mill with TiAlN-coated octagonal carbide inserts and run the spindle at 400–500 rpm.

The first pass is deliberately heavy, removing 2.5–3.5 mm. Feed per tooth is 0.25 mm, while table feed is 600–800 mm/min. Coolant is not used; instead, 0.6 MPa compressed air blows chips away. Dry cutting often extends insert life by about 15%.

For thin plates under 20 mm, a large face mill may pull the plate out of shape. In such cases, a 63 mm cutter is used with only 1 mm depth of cut, and spindle speed is increased to 800 rpm. An electromagnetic chuck with breathable cloth underneath helps stabilize the plate. After every five passes, the plate must be flipped to avoid bowing.

Once all six sides are milled, experienced workers check a 1000 mm plate by comparing its diagonals. The difference must not exceed 0.5 mm. Thickness tolerance is held to +0.1 to +0.15 mm, and width oversize to 0.2 mm maximum. The datum face must stay within 0.03 mm per meter.

· Decarburized black skin removed per side: 2.5 mm

· Finish allowance reserved per side: 0.15–0.2 mm

· Surface roughness after milling: Ra 3.2 μm

· Edge chamfer: C1.5

· Squareness error between adjacent faces: below 0.05 mm

For mold bases with tighter requirements, large reciprocating surface grinders are used after rough milling. Operators select 46-grit or 60-grit white aluminum oxide wheels running at 30–35 m/s. Table speed is 15–20 m/min.

Depth of cut is extremely small. During rough grinding, each pass removes about 0.015 mm; during finish grinding, only 0.005 mm or even 0.002 mm. Coolant flow is 40–50 L/min, keeping heat away from the workpiece. The finished surface reaches Ra 0.8 μm.

Ground flat stock can achieve thickness accuracy of 0.02 mm. Over 1000 mm, thickness difference between the two ends stays below 0.015 mm. The wheel spindle is dynamically balanced to keep runout below 0.003 mm. Operators also make two idle spark-out passes after feed stops to remove residual stress.

If more than 15% of one face of a P20 block is removed, internal stress can cause warping. On a 100 mm thick plate, removing 20 mm from one side may create upward bowing of 0.3–0.5 mm. Uneven stock removal almost always leads to distortion.

In such cases, the block is placed in a 10-ton trolley furnace for stress relief. Heating is slow, only 50°C per hour, up to 550°C. For a mold base measuring 800 × 600 × 150 mm, a soaking time of about 6 hours is required. The power is then shut off and the furnace remains closed until the temperature falls to 200°C.

· On a granite table, height difference stays below 0.02 mm

· Check with a 0.02 mm feeler gauge and 1000 mm straightedge

· Ultrasonic thickness gauge at nine points: max difference below 0.01 mm

· Shore hardness readings: variation below 1 HRC

After heat treatment, the plate should sit in an inspection room at 20°C and 50% humidity for 24 hours. A Hexagon CMM can then probe around 120 points, and the software compares the 3D profile against the original drawing.

Turning and Milling Parameters

At 28–34 HRC, P20 is already too hard for ordinary HSS inserts in lathe work. For outsourced round mold components, operators almost always switch to CVD-coated carbide inserts. The tool holder is set carefully, with a principal cutting edge angle of 75° to direct cutting forces toward the spindle.

For rough turning of the outer diameter, cutting speed is kept at 80–110 m/min. Depth of cut is 3–5 mm, and feed is 0.3–0.4 mm/rev. Chips should form blue-purple C-shaped curls. If they turn red, the cutting zone has likely exceeded 600°C, and the tool tip will fail quickly.

Daily shop-floor consumable standards are usually fixed:

· CVD-coated insert life: about 45 minutes per cutting edge

· Emulsion coolant concentration: 5%–7%

· Tailstock clamping pressure: 2.5 MPa

· Three-jaw chuck clamping force: 35 kN

The last 0.5 mm is reserved for finish turning. A PVD-coated insert is then used at 130–160 m/min, with 0.2 mm depth of cut and 0.1 mm/rev feed. With coolant directed onto the tool tip, surface finish can reach Ra 0.8 μm.

To prevent inexperienced operators from changing settings arbitrarily, many shops post baseline turning data next to the machine. Following those values reduces scrap risk significantly.

For block material on a CNC machining center, flattening is done with face mills over 100 mm in diameter. During rough milling, spindle speed is 250–350 rpm, giving a cutting speed of about 70–90 m/min. The cutter typically carries 6–8 round carbide inserts, each taking 0.2 mm per tooth, producing a deep but stable cutting sound.

Depth of cut is set to 3 mm, and table feed is 400–500 mm/min. Chips scatter to both sides like heavy rain. Water coolant should not be used, because rapid thermal cycling can crack the insert edge. Instead, 0.5 MPa compressed air is directed through an 8 mm outer-diameter hose.

Tool maintenance is manual and must be handled carefully:

· Insert screw torque: 3.5 N·m

· Face mill axial runout: below 0.02 mm

· Pull stud wear: mandatory replacement every 6 months

· Spindle taper cleaning: wipe with lint-free cloth before every tool change

For slotting, climb milling is preferred. The chip is thickest at entry and thins to about 0.02 mm at exit, helping press the workpiece against the table. Conventional milling, by contrast, causes the insert to rub and skid across the hard P20 surface, dramatically shortening tool life.

When switching to a 20 mm end mill for cavity machining, tool overhang should not exceed 3× diameter, or 60 mm. Excessive overhang causes chatter. Spindle speed is typically 1500 rpm, or about 90 m/min. Depth of cut per pass is 1.5 mm, and radial engagement is 60% of tool diameter.

Operators can often judge tool wear by sound. When the low cutting tone turns into a sharp squeal, the edge is usually worn flat. Under magnification, once flank wear exceeds 0.3 mm, the insert must be replaced immediately. If not, the P20 surface can work-harden to as much as 40 HRC.

The next tool then struggles to bite, causing slipping and heat buildup. In such cases, experienced machinists reduce spindle speed by 30% and increase depth of cut to above 0.5 mm, forcing the tool through the hardened surface layer and into sound material. For finishing complex curved surfaces, holder clamping accuracy becomes critical.

Deep-Hole Drilling and Cooling

Internal cooling channels in P20 mold steel are usually drilled on dedicated gun-drilling machines. For a 10 mm hole deeper than 200 mm, a standard twist drill is no longer practical. In steel around 30 HRC, every additional millimeter of drilling depends on high-pressure cutting fluid to flush chips out.

Before drilling, the operator uses a standard carbide drill to make a pilot hole about 15 mm deep. Its inner diameter must be 0.02–0.03 mm larger than the gun drill diameter, providing a stable guide. Without it, a 1.5-meter hollow drill tube may whip like a twisted rod as it starts rotating.

If wear in the guide bush at the front of the gun-drilling spindle exceeds 0.015 mm, the center of a blind hole drilled to 500 mm depth may drift by as much as 1.5 mm.

For P20, spindle speed is generally 2500–3000 rpm, corresponding to a cutting speed around 80 m/min. Feed is 0.03–0.05 mm/rev.

Tool life depends entirely on high-pressure oil supply:

· Pump pressure: 40–80 bar, depending on hole diameter

· Minimum flow rate: at least 25 L/min, with deeper holes requiring more

· Oil viscosity at 40°C: 15–20 mm²/s

· Temperature control: circulating oil kept below 35°C

· Filtration: return line fitted with 10 μm paper filter elements

A clean, continuous rustling sound indicates chips are being evacuated properly. Standard chips from P20 should curl into “6” or “9” shapes about 3–5 mm long.

At a drilling depth of 800 mm, deviation becomes the biggest concern. Steel naturally contains areas of uneven hardness. If the drill tip encounters a local hard zone just 2 HRC higher than the surrounding material, it tends to deflect toward the softer side.

When drilling through from both ends, if the mismatch at the meeting point exceeds 0.2 mm, the cooling water can form turbulence there during injection molding, causing an immediate pressure drop through the entire circuit.

Machine technicians can only correct this by grinding the drill geometry carefully. The outer cutting edge is ground to a principal angle of 20°, the inner edge to 30°, with an auxiliary angle of 8–10°. These angles push cutting force toward the centerline and help the drill stay straight.

Under high temperature and pressure, carbide tips wear very quickly:

· Gun drill life: a premium imported drill usually lasts for 15–20 meters of total drilling length in P20

· Flank wear limit: 0.2 mm, measured on a 50× optical projector

· Regrinding stock removal: 0.5–1 mm from the tip each time; one drill can usually be reground up to 10 times

· Backup inventory: at least 3 reconditioned drills of the same size should be kept on hand for deep waterline drilling

If the drawing calls for water channels above 20 mm in diameter, the chip space of a gun drill is no longer sufficient. The workshop then switches to a BTA internal chip-removal system. With 15 MPa high-pressure oil forced through the gap between the hole wall and the drill tube, chips are drawn back through the hollow drill body, making blockage far less likely.