P20 Tool Steel Heat Treatment

The two heat treating routes for P20

P20 is a chromium and molybdenum plastic mold steel, and the tonnage sold in the market goes out already at working hardness. Two situations still put the grade in a furnace. A block that arrives in the annealed or forged condition has to be hardened and tempered to reach that working hardness, and a mold surface that has to resist abrasion while the body of the block stays tough is carburized. The reference card for the grade covers both routes, and the tables below follow the order the card sets them in.

The figures on this page come from one card in a shop reference book rather than from a specification, so they read as a working recipe. The hardening temperature and the tempering response describe a P20 section hardened directly from the annealed or forged condition. The carburizing block under them is a separate route with its own austenitizing temperature, its own cooling step and its own hardness table for the case and the core.

Chemistry of P20

Elements the reference card lists, percent

Source, heat treating reference information for P20, printed page 195.

Carbon, manganese, silicon, chromium and molybdenum are the five elements the card lists with a figure against them. Vanadium, tungsten, cobalt and sulfur appear on the card with no figure and are left out of the table rather than shown as a zero. Every figure is a single value and not a specification range.

Chromium at 1.65% and carbon at 0.30% are the two figures that set the working hardness and the wear resistance of P20, and molybdenum at 0.40% carries the hardenability that lets an oil quench reach through a mold block. Manganese and silicon, at 0.75% and 0.50%, are the deoxidation and solid solution additions. The card lists vanadium, tungsten, cobalt and sulfur with no figure against them, and those rows are left out of the table rather than printed as a zero. Composition data in the table is for general reference only, actual values vary by standard, mill and heat number, so confirm the figures against the material test certificate of the heat or ask Aobo Steel.

Hardening P20 from the annealed condition

Preheat, hardening temperature and quench

Source, heat treating reference information for P20, printed page 195.

* = if used on larger mass parts. The card gives one hardening temperature for the grade and an oil quench. Another chart on this site puts P20 hardening in the 1600 to 1650 °F (870 to 900 °C) band, and the card gives that higher 1600 °F (870 °C) figure for the carburizing route set out below.

The card gives the grade one hardening temperature of 1525 °F (830 °C) with an oil quench, and the 1200 °F (650 °C) preheat is marked for larger mass parts. A mold block that goes into the quench in one piece carries enough mass to need that step. The same card sends the carburizing route through a higher austenitizing temperature of 1600 °F (870 °C), which is set out further down this page, so the two temperatures belong to two different routes through the furnace.

Tempering response of P20

Hardness left by each tempering temperature

Source, heat treating reference information for P20, printed page 195.

The card steps the tempering temperatures by 200 °F to 1000 °F and then by 100 °F and 50 °F. It prints one Rockwell C figure for each temperature with no range.

The line starts at 51 HRC as quenched and falls steadily as the tempering temperature rises. A temper at 600 °F (315 °C) leaves 47 HRC, 1000 °F (540 °C) leaves 39 HRC, and by 1250 °F (675 °C) the hardness is down to 21 HRC. There is no second rise anywhere on the line, which is what a grade without secondary hardening carbides looks like. For a block hardened from the annealed condition, the practical choice sits in the upper half of the table and the top three temperatures are past the point where a mold can hold its shape.

Carburizing P20

Gas carburizing, cooling and quench cycle

Source, heat treating reference information for P20, printed page 195.

The card places the carburizing block immediately under the hardening card and gives it its own austenitizing temperature, its own cooling step and its own quench.

Carburizing raises the carbon at the surface of the block while the core keeps the composition it started with, so the mold face resists abrasion and the body behind it stays tough under load. The card runs the gas carburizing step at 1600 °F (870 °C), cools the load in the furnace to 1475 °F (800 °C) and then quenches it in oil and tempers it. The furnace cool is what keeps the core from being hardened to the section thickness, and it is the reason the case and the core come out of the same tempering cycle at two different hardness levels.

Hardness of the case and the core after tempering

Source, heat treating reference information for P20, printed page 195.

The source prints the second row as 650 °F/354 °C. The Fahrenheit figures above and below it step by 50 °F and 650 °F converts to 343 °C, so the Celsius figure is printed here as 343 °C and the source value is recorded so the row can be checked against the book.

The case holds 58 HRC at a 600 °F (315 °C) temper and 52 HRC at 900 °F (480 °C), and the core comes down from 47 HRC to 40 HRC across the same band. A temper between 700 and 800 °F (370 and 425 °C) leaves the case in the mid fifties with the core still at 43 HRC or above, which is the window most surfaces are specified in.

Carburizing time and the case depth it gives

Source, heat treating reference information for P20, printed page 195.

The depths are printed on the card as average values, with no tolerance stated. The inch and millimetre figures are reproduced as the card gives them.

The card pairs four carburizing times with an average case depth. Four hours gives 0.030 in (0.762 mm), eight hours 0.042 in (1.0668 mm), twelve hours 0.055 in (1.397 mm) and sixteen hours 0.062 in (1.5748 mm). The rate falls away as the time goes up, because carbon has further to diffuse with every extra hour, so the step from eight hours to sixteen doubles the furnace time for 0.020 in (0.51 mm) more case. A mold face specified at 0.030 in of case saves a full day in the furnace against one specified at twice that depth.

Related reference data

The condition P20 arrives in from the mill is described on the annealed condition page, and the grade itself with its wall thickness range is on the P20 tool steel page. The choice between this grade and the hot work steel for a mold that sees higher temperature is set out on the H13 and P20 selection page. Hardening and tempering bands across the tool steel groups are on the tool steel hardening and tempering chart, tempering bands alone are on the tool steel tempering chart, and the specification ranges and equivalents for the grade are on the tool steel composition chart.

Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, second edition, Hanser). The hardening card, the tempering response, the carburizing cycle, the case and core hardness and the case depths for P20 come from the heat treating reference information on printed page 195.