H19 Tool Steel Heat Treatment

The chemistry, the hardening cycle and the tempering line the source card prints for H19 heavy duty hot work tool steel. A reference summary of published practice and not an Aobo Steel specification.

H19 in the hot work group

H19 is a hot work tool steel that carries tungsten and cobalt on top of chromium, and that is what separates it from the chromium grades H11, H12 and H13. The reference card prints 4.26 percent tungsten and 4.16 percent cobalt against 4.12 percent chromium, so the grade is built for heat resistance rather than for the hardness the cold work grades are bought for. The source text places it in die casting, where its resistance to heat checking is the reason the grade is named at all, and it adds that the steel can give exceptional wear in high temperature work. It is also the grade the same source calls the better all round choice beside H21, because H19 keeps more hardness and more wear and takes shock better.

The card is shop reference information rather than a specification. It prints one preheat step, one hardening temperature and two quench media, then a tempering table in Rockwell C with a single figure on each row and no tolerance band anywhere. A first run of parts from a new heat of steel is worth treating as a trial before the cycle goes onto production work.

Chemistry of H19

Elements the reference card lists, percent

Source, heat treating reference information for H19, printed page 188.

The card prints sulfur with no figure against it, so that row is left out rather than printed blank. The card gives a single figure for each element where a standard gives a range, and the figure on each row here falls inside the range the tool steel composition chart on this site carries for H19, which writes the chromium range as 4.00 to 4.75 percent, the tungsten range as 3.75 to 4.50 percent, the vanadium range as 1.75 to 2.20 percent and the cobalt range as 4.00 to 4.50 percent against UNS T20819. Composition data 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.

Chromium at 4.12 percent is the figure the heat resistance rests on, and it is lower than the chromium in the H11 and H13 grades, which is why this steel carries tungsten and cobalt as well. Tungsten at 4.26 percent and cobalt at 4.16 percent are the two elements behind the hot strength, and cobalt is the addition that raises the temperature at which the steel still holds its hardness. Vanadium at 2.19 percent is high for a hot work grade, and it is the element that forms the hard carbides which keep the wear resistance at working temperature. Molybdenum at 0.46 percent works alongside the tungsten. Manganese at 0.31 percent and silicon at 0.21 percent are the usual minor additions that come with the deoxidation practice. Carbon at 0.42 percent is on the low side of the tool steel range, and that is deliberate in a die steel, because carbon buys hardness at the cost of toughness and a die that cracks is worse than a die that wears. 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 H19

Preheat, hardening temperature and quench

Source, heat treating reference information for H19, printed page 188.

The card prints one preheat step, one hardening temperature and two quench media for the grade, with no section size condition against any of them, so the figures stand for the load as written. Both media named on the card cool faster than air, which is the point of the grade rather than an accident of the card.

The load goes in at 1500 °F (815 °C) for the preheat, then up to 2150 °F (1175 °C) for austenitizing, followed by an oil or a salt quench. Both media named on the card cool faster than air, and that is the difference between this grade and an air hardening tool steel, so the section has to be able to take the faster cool. Salt is the medium a commercial heat treater reaches for on a die block, since it cools evenly and leaves a clean surface, and oil is the older route printed beside it. A heavy die section and a fast quench are the combination that produces distortion and quench cracks, which is why the preheat step matters more here than it does on a small part. The card prints one hardening temperature and no soak time, so the time at temperature has to come from your heat treater’s practice rather than from this table.

Tempering response of H19

Hardness left by each tempering temperature

Source, heat treating reference information for H19, printed page 188.

The card prints a Rockwell C figure on every row and no tolerance band against any of them. The line falls from the as quenched figure over the first four rows and then climbs to the highest figure on the card at 1100 °F (595 °C), which is the top of the range the card prints.

The line opens flat and then climbs. H19 leaves the quench at 53 HRC, and the first four tempering temperatures printed on the card, 600 °F (315 °C) through 900 °F (480 °C), take the reading down to 52 and 51 HRC. From there the line turns up, to 56 HRC at 1000 °F (540 °C) and 57 HRC at 1100 °F (595 °C), the highest figure on the card and the top of the range it prints. That rise is secondary hardening. The tungsten, vanadium and cobalt carbides come out of solution during the temper and put hardness back into the steel, which is why a hot work die is tempered at the top of this range rather than at the bottom. A die that has to hold its hardness at 1000 °F (540 °C) in service is tempered close to that temperature in the first place. Nothing is printed above 1100 °F (595 °C) on this card, so the tail of the curve sits outside the reference data on this page.

Related reference data

H19 sits with the other hot work grades on this site. The grades beside it are set out on the H11 tool steel heat treatment page, the H13 steel heat treatment page, the H21 tool steel page and the H10 tool steel heat treatment guide, and the hot work tool steel catalog collects the family. Working ranges across the tool steel groups are on the tool steel tempering chart, which carries a row for H19, the hardness of the same grades at temperature is on the tool steel hot hardness chart, which carries an H19 row, and chemistry for every grade is on the tool steel composition chart. Selection by application is covered on the tool steel selection by application page and the die casting case on the why H13 steel is used in die casting page.

Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, second edition, Carl Hanser Verlag, 2009). The chemistry, the hardening cycle and the tempering response for H19 come from the heat treating reference information on printed page 188.