CS251553B1 - Method of heat treatment of carbon steel products and semi-finished products - Google Patents

Abstract

The solution concerns a method of heat treatment of products such as thick sheets, bars, pressings, forgings or castings made of carbon steels with a content of up to 0.35% by weight of carbon and alloy steels with a total content of alloying additives of up to 11% by weight. Its essence is to cool the steel at a rate of 5 to 1,500 °C.h~l to an intercritical temperature, which is located in the area defined by the temperatures Ac3 -3 °C and AC1 +5 °c. The product is maintained at this temperature for 0.1 to 12 h. This is followed by cooling to room temperature, or reduced to -196 °C. Depending on the type of steel processing, tempering is then performed at a temperature lower than the critical temperature by at least 15 °C.

Description

The invention solves a method of heat treatment of rolled products, forgings or castings made of carbon steels with a content of up to 0.35% by weight of carbon and alloy steels with a total content of alloying additives of up to 11% by weight to achieve high resistance to brittle fracture and a low tendency to temper brittleness.

Until now, heat treatment has been carried out by heating rolled products, forgings or castings made of carbon and alloy steels with a total content of alloying additives up to 11% by weight to the austenitization temperature, i.e. to a temperature higher than the critical temperature of the steel, followed by cooling in air, in a gas-air stream or in a liquid medium, e.g. water, oil. When continuous cooling from the austenitization temperature is used, the austenite is converted into the resulting microstructure, which determines the useful properties of the steel. So far, only different cooling rates from the austenitization temperature have been used to influence the final properties. This limits the achievement of the required strength and at the same time the necessary resistance to brittle fracture. In addition, steel processed in this way becomes embrittled during the inevitable subsequent tempering due to the development of tempering brittleness. This form of brittleness also manifests itself in components processed in this way that have been operated for a long time at elevated temperatures.

To improve the resistance to temper embrittlement, a procedure called three-stage heat treatment has been developed. This method consists of three thermal operations, namely heating to a temperature above the critical temperature A^ of the given steel and quenching from this temperature to full cooling of the processed product, followed by a second heating to a temperature between the critical temperatures A^3 and ^C1 ^and quenching. The third operation is tempering at a temperature below A^.

However, this expensive process, requiring significant furnace capacity and high electrical energy consumption, does not ensure the minimum required strength properties with sufficient margin.

The above disadvantages are eliminated by the method of heat treatment of products and semi-finished products made of carbon steels with a content of up to 0.35% by weight of carbon and alloy steels with a total content of alloying additives of up to 11% by weight according to the invention, using heating of the steel above the transformation temperature A^. Depending on the type of steel being processed, tempering is performed after heat treatment at a temperature lower than the critical temperature A^, by at least 15 °C.

The essence of the invention is the cooling of steel at a rate of 5 to 1,500 °C.h to an intercritical temperature. This temperature is in the range defined by the temperatures A^j -3 °C and +5 °C. The product is maintained at this temperature for a period of 0.1 to 12 h. This is followed by cooling to room temperature, or to a temperature reduced to -196 °C.

The heat treatment according to the invention achieves a very favorable combination of high strength properties and favorable toughness and resistance to brittle fracture, suppressing the tendency to temper brittleness, which is unattainable by other processing methods.

The heat treatment method according to the invention is generally carried out by heating the product above the critical temperature A^. The critical temperatures A _C1 and A _c3 are the transformation temperatures of the steel of the product, given in the steel data or determined experimentally, or calculated from the chemical composition of the steel. In hot pressing or forging, heating to the pressing or forging temperature can be advantageously used to carry out the treatment according to the invention, if the temperature at which the pressing or forging is completed is in the intercritical range. This is followed by cooling the steel to the intercritical range.

The intercritical region is a two-phase region of austenite and ferrite, bounded by the critical temperatures A| and A^. By including a dwell in the intercritical region during the processing according to the invention, a certain volume fraction of austenite is converted into ferrite, so that during the dwell at a temperature between A^ A^i, austenite and ferrite simultaneously exist in the microstructure of the steel. This simultaneous occurrence of both phases in a suitable volume amount allows carbon and alloying and accompanying additives and impurity elements to be distributed between both phases in the steel according to their mutual solubility. This creates the conditions for the formation of a resulting advantageous microstructure, which gives the steel high strength, favorable deformability and increased resistance to brittle fracture compared to

25155*3 with conventionally heat-treated steel. During conventional heat treatment, harmful impurities are mixed at the grain boundaries, which subsequently causes tempering brittleness. Such mixing of impurities in steel is prevented by including a dwell time in the intercritical region during heat treatment according to the invention, which thus suppresses the occurrence of tempering brittleness of the steel of the product. After an isothermal dwell time in the intercritical region, the product is cooled at a rate necessary to achieve the desired microstructure. Depending on the type of steel being processed, tempering at a temperature below Αβ is included to further increase toughness.

An exemplary embodiment of the heat treatment according to the invention is schematically shown in the attached graph, showing the course of the heat treatment of steels as a function of time:temperature, where the x-axis shows the temperature and the y-axis shows time. In the graph

A indicates the heating time period,

B indicates the austenitization time period,

C indicates the time period of controlled cooling,

D indicates the delay time,

E indicates the cooling time period and

P indicates the tempering time period.

The specific procedure for heat treatment of products and semi-finished products according to the invention is illustrated in more detail in the following examples.

Example 1

A forging made of steel with 0.23% carbon, 1.14% manganese and 0.12% vanadium, for which A = 730 °C and A^ = 830 °C, was heated to an austenitization temperature of 910 °C. This was followed by cooling at an average rate of 30 °C.h to a temperature of 760 °C. After a two-hour stay at this temperature in the intercritical region, the forging was quenched in water at room temperature. Tempering was carried out at 620 °C for 4 h. The following properties were obtained by this treatment:

yield strength R _{Λ n} = 515 MPa p0.2 strength R^ = 668 MPa ductility A,- - 23 % contraction Z = 66 % notch toughness KCV = 140 J.cm at 20 °C

Long-term annealing at a temperature of 480 °C of the forging thus processed did not cause detectable temper brittleness.

For comparison, the properties of steel processed in the previous method, i.e. without controlled cooling, were determined. The yield strength R . _ = 360 MPa ^p0 ' ² -2 o notch toughness KCV = 84 J.cm at 20 C was achieved.

Example 2

Another example is the heat treatment of a thick sheet of steel with 2.25% chromium and 1% molybdenum, containing 0.12% carbon. The critical temperatures of the steel were A^j = 865 °C, A^ = 792 °C.

After austenitization at 930 °C, the sheet was cooled at a controlled rate of 45 °C.h to a temperature of 840 °C, which was between A^ and A^. After a 12-h delay at this intercritical temperature, the sheet was quenched in water to room temperature and then subjected to the usual tempering at 680 °C. This heat treatment yielded the following values: i yield strength strength ductility p0.2

568 MPa 667 MPa 22%

contraction From = 80 % notch toughness KCV = 260 T -2 J.cm at 20°C ductile transition temperature to brittle fracture ⁵⁰ = -65 °C fracture toughness ^To cJ = 216 MPa.m ² at 60°C. Processing for the purpose of intentional invocation tempering embrittlement did not arrive in this way thermally

of the sheet metal to embrittlement.

For samples of the same sheet metal processed using the previous method, the following values were achieved:

yield strength fracture toughness ductile to brittle fracture transition temperature ^R p0, ^K cJ ^fc 50 = 320 MPa = 72 MPa. , ^m2 at -60°C = +20 C. : k 1 and d 3 A forging made of steel containing 0.17% carbon, 1.05 % manganese and 0.22% silicon

The temperatures were A^ = 846 °C and A _cl = 716 °C, it was processed in the following way: after austenitization at 880 °C, cooling was carried out at an average rate of 12 °C to a temperature of 750 °C. After a 0.1 h stay at this temperature, the forging was cooled by a stream of air at room temperature. This processing achieved the values:

yield strength ductility contraction notch toughness ^R p0.2 ^{= 392 MPa}

R = 546 MPa m

And ₅ = 29%

Z = 72%

KCV = 95 J.cm ^-2 at -40 °C.

The following properties were achieved for samples made from the same steel, but without controlled cooling and holding at the specified temperature*:

yield strength R _n ~ = 280 MPa ^p0 ' ² -2 o notch toughness KCV = 12 J.cm at -40 C.

Example 4

A thick sheet of steel containing 1.6% manganese, 0.8% nickel, 0.13% vanadium and 0.14% carbon was cooled from an austenitizing temperature of 89J °C at an average rate of 1,500 °C.h to a temperature of 725 °C, which is 15 °C above the critical temperature A^ of steel of the given chemical composition. The sheet was maintained at a temperature of 725 °C for 8 h, after which it was cooled to a temperature of -196 °C and tempered at 650 °C. The following useful properties were achieved by the described method:

yield strength ductility contraction notch toughness fracture toughness

R _{Λ o} =570 MPa p0.2

R = 680 MPa m

A_ = 22% b

Z = 68%

KCV = 68 J.cm ^-2 at -60 °C

K _cJ = 162 MPa.m ^-2 at -100 °C.

The properties of samples made from the same sheet metal, but without the described heat treatment, were as follows:

yield strength notch toughness fracture toughness

R _{Λ n} p0.2

KCV

410 MPa

J.cm' ² at -60 °C 58 MPa.m ² at -100 °C.

Claims (1)

Process for heat treatment of carbon steel products and semi-products up to 0.35% by weight of carbon and alloy steels with total alloying content up to 11% by weight, consisting of heating the steel above the conversion temperature and possibly tempering at a temperature of at least 15 ° C than the critical temperature and, ^ characterized in that the article is cooled at 5 to 1,500 ° Ch ^_ l mezikritickou temperature which is within the area defined nechází temperatures N- -3 ° C and + 5 ° C, at this temperature the product is maintained for 0.1-12 h and then cooled to room temperature or reduced to -196 ° C.