CN1545564A

CN1545564A - Method and device for quenching steel in pressurized air

Info

Publication number: CN1545564A
Application number: CNA028163850A
Authority: CN
Inventors: F; F·弗罗蒙特; M·干托伊斯; D·吉莫曼
Original assignee: Serthel
Current assignee: Serthel
Priority date: 2001-06-21
Filing date: 2002-06-20
Publication date: 2004-11-10
Anticipated expiration: 2022-06-20
Also published as: FR2826374B1; WO2003000939A1; US20050000597A1; DE60217344T2; CN1289695C; MXPA03012026A; DE60217344D1; EP1404882B1; FR2826374A1; EP1404882A1; ATE350497T1

Abstract

The method for quenching a steel charge after carburizing or carbonitriding is carried out at atmospheric pressure and comprises the following steps: the charge is extracted from a treatment furnace (1) at a temperature ranging from 750 DEG C to 1100 DEG C; b) the charge is transferred to a quenching cell (3); c) a quenching fluid is introduced at a pressure which is higher than atmospheric pressure; d) the charge is cooled to a temperature of less than 400 DEG C. According to the invention, the fluid is primarily composed of air, and the part is brought to a temperature of at least 400 DEG C, whereby an oxide layer preventing decarburization is formed in the time period starting from the moment when the charge exits from the furnace.

Description

Method and apparatus for quenching steel with pressurized air

The present invention relates to a method of quenching steel with pressurized air, which can ensure the same metallurgical characteristics as oil quenching, and particularly can prevent decarburization of the steel surface after carburizing or carbonitriding treatment at atmospheric pressure.

The pressurized air quench process has numerous advantages over the liquid quench process, particularly because the quenched parts are clean and dry.

There are many papers approved on quenching steel with pressurized gas and many apparatuses exist that can be distinguished by the structure of the quenching chamber and the nature of the gas used, with the aim of obtaining the highest quenching efficiency possibility, i.e. the maximum cooling rate, in order to equalize the cooling rates obtained with various liquids, such as oils, aqueous solutions or molten salts. For this purpose, one increases the gas mass flow circulating over the surface of the part to be cooled and increases the gas velocity and pressure.

The most commonly used gas is nitrogen, which, because of its chemical inertness and relatively acceptable cost, does not need to be recycled after each quenching operation. It is often used at pressures between 1 bar and 10 bar.

Other inert gases have been proposed for use. As such, european patent 0313888 suggests the use of light gases such as helium and hydrogen at relatively high pressures (equal to or greater than 20 bar). The use of these light gases aims at: at the same performance, both circulation flowand thrust of the turbine are ensured. However, these gases are either particularly expensive and therefore impose recirculation, such as helium; or particularly hazardous, such as hydrogen.

All the gases suggested by the prior art, which differ in their physical characteristics: molecular weight, thermal conductivity, etc., but they are chemically inert with respect to steel subjected to high temperatures prior to quenching. The aim is to avoid oxidation and surface decarburization of the steel.

The apparatuses for quenching with pressurized gas all contain turbines that allow the gas to circulate over the parts. The gas is then cooled by passage over a heat exchanger and then re-injected onto the part feed.

The applicant determined that the use of pressurized air to achieve quenching is a goal because air is low cost and disposable. However, its utilization also poses a major problem. Oxygen is chemically very reactive with steel at high temperatures and often causes industrially unacceptable surface decarburization.

In fact, it is known that maintaining the high temperature of the steel in air, in particular between 700 ℃ and 1100 ℃, results in the formation of an oxide layer, commonly referred to as "scale", which grows very rapidly. After the scales are exposed to air for a few minutes, their thickness reaches tens of micrometers. The formation of this oxide layer is also accompanied by decarburization of the steel surface, which is detrimental to the metallurgical characteristics and mechanical properties of the steel.

The invention relates to a method and a mode of operation thereof, which allow the quenching of a steel subjected to carburizing or carbonitriding treatment at atmospheric pressure, at a temperature comprised between 700 ℃ and 1100 ℃, by means of pressurized air, such quenching resulting in a metallurgical behaviour similar to that of an oil quench starting from the surface of the steel, i.e. preventing all the surface decarburization overall.

According to the invention, the steel feed quenching method comprises the following steps:

-extracting the charge from a carburizing or carbonitriding treatment furnace operating at atmospheric pressure at a temperature between 750 ℃ and 1100 ℃,

-transferring the feed towards the quenching chamber,

-injecting a quenching fluid at a pressure higher than atmospheric pressure, causing the fluid to circulate in the chamber,

-cooling the feed to a temperature below 400 ℃,

the method is characterized in that: the fluid, which is mostly constituted by air, brings the temperature of the piece to at most 400 ℃ during the time from the exit from the furnace and during this time forms an oxide layer which prevents decarburization of the steel.

In this description, it is considered that since the carbon content measured in the thickness of 60 μm from the surface of the steel is still unchanged, it means that the steel does not suffer decarburization.

The method of the invention can be used, in particular, for the quenching of carbon steels, carburized steels, carbonitrided steels and tool steels with pressurized air.

The present invention is based on the following observations. Carbon steels oxidize at temperatures near 900 ℃, resulting in the formation of a vickers or wustite-type (substoichiometric FeO) oxide layer. The thickness of this oxide layer increases regularly with time, reaching 10 μm to 12 μm after 1 minute in still air. It is estimated that the vickers oxide layer forms a thickness of 1 μm after 5 seconds in air. At the start temperature of quenching of normal carburized or carbonitrided steel, 870 ℃, the oxide layer thickness is about 4 μm after 20 seconds.

On the other hand, it is considered that after 20 seconds, and in the presence of an atmosphere whose latent carbon is less than 0.1% at 870 ℃, the layer thickness affected by decarburization is about 20 μm. This decarburisation occurs when carbon monoxide can be released in the atmosphere. This is caused by the reaction The result is that C is carbon dissolved in the steel.

Thus, decarburisation of the steel surface layer is avoided by forming a barrier against any release of carbon monoxide. Due to the short disposable time, it is necessary to form this barrier layer quickly.

The present inventors have recognized that a vickers type oxide layer can function to a sufficient extent to adhere to the steel surface.

An oxide layer of 1 μm thickness is sufficient to prevent the formation of CO in a significant way.

If there is no rapid temperature change (thermal shock), a good adhesion of the oxide layer is obtained as long as the thickness of the oxide layer remains below 10 μm to 12 μm, preferably below 4 μm to 5 μm. One arranges a period of 20 to 40 seconds at a temperature close to the usual quenching temperature, during which the iron oxide layer formed becomes a barrier against any release of carbon monoxide, thus avoiding any significant decarburisation on the surface of the steel.

According to another characteristic of the invention, the feed consisting of the steel part kept at the quenching starting temperature is transferred into the quenching chamber in air, the quenching being started by bringing the surface temperature of the part to below 600 ℃ in a period of less than 40 seconds, preferably in a period of less than 20 seconds.

The quenching fluid in the quenching chamber, at a pressure of up to 40 bar, starts up in a few seconds at a speed of up to 20 m/s. The quench fluid consists primarily of air. One may add other components to the air that aim to improve heat transfer.

During quenching, the part surface temperature was measured. The temperature is reduced to a value lower than 700 ℃ in less than 2 seconds; (ii) decreases to a low value of less than 600 ℃ in less than 4 seconds; and to below 400 c in less than 10 seconds. At temperatures below 600 c, the carbon diffusion rate in the steel is so slow that decarburization is not observed.

Further advantages and characteristics of the invention may be seen in the description, accompanied by the appended drawings, and thus non-limiting embodiments of the invention, in which:

fig. 1 shows a schematic view of an apparatus according to the invention.

FIG. 2 shows the carbon concentration profile of the surface layer of case hardened steel and quenched steel, on the one hand by pressurized air quenching according to the invention and on the other hand by oil quenching.

FIG. 3 shows the microhardness profiles of the surface layers of case hardened steel and quenched steel, on the one hand by pressurized air quenching according to the invention and on the other hand by oil quenching.

The apparatus of fig. 1 comprises a well-known processing furnace.

The apparatus relates to a continuous or batch carburizing or carbonitriding furnace 1, such as a shaft furnace, a bell furnace, which carries out thermochemical treatment at atmospheric pressure.

At several distances on the same platform, a quenching chamber 3 using gaseous fluid is present. The quenching chamber is sealed closed by a shutter 31. Inside the chamber, rotors such as centrifugal and helical blades 33 are provided, called turbines, which are driven by an electric motor 35. The turbine functions to flow the gas contained in the chamber for quenching. The gaseous quench fluid is directed by guide 37 in the direction of the illustrated arrow or in the opposite direction. The gas passes through a heat exchange device 39. For quenching, the chamber is sealed closed after a charge of block a4, shown in phantom, is placed in the chamber. The chamber is charged until the desired pressure between 1 and 40 bar is reached, while the air is driven by the turbine 33. Preferred pressures are greater than 3 bar, and the gas pressure may be comprised between 5 and 20 bar, in particular according to the nature of the part to be treated. Several suitable deflectors inject the cold gas relatively toward feed a 4. After extracting the heat from the feed, the cold gas is directed to a turbine. It passes through a heat exchanger and is returned to the quench temperature.

In the embodiment shown in fig. 1, both the exchanger and the turbine are arranged inside the chamber, but may also be arranged outside the chamber.

The feed to be treated may consist of, for example, gears or shafts.

An arm manipulator 40 is placed between the furnace and the chamber. Its function is to receive the furnace feed and to place the feed on the chamber supports, the different positions of the feed being shown between a0 and a4, a0 being the position in which the feed is still inside the furnace and a4 being the position in which the feed is on the chamber supports during quenching.

The arm manipulator comprises a platform 41 which is rotatable about a vertical axis and displaceable in height. The platform 41 comprises an arm 43 that can move in a horizontal plane and receive the feedstock to be processed. The arm is movable between an outer position and an inner position, the arm in the outer position being adapted to receive and position the feed material; the arms, in the inner position, allow the charge to be loaded into the protective cover 45.

The working cycle is as follows:

-placing the feed material in the furnace 1 at position A0 and at the treatment temperature. The furnace gate is opened.

The arm 43 faces the oven door opening. The arm is extended to a position where the feed is just placed. The arms are retracted so that the feed is located just below the shield 45 at position a 1.

The platform is pivoted so that the feedstock is positioned right facing the support 31 in position a 2.

-extending the arms to place the charge on the stand 31 at position a 3.

-moving the carriage until just inside the chamber. The feed enters position a 4. The temperature of the feed material does not drop significantly after it exits the furnace.

-starting the quenching process.

According to the invention, the quenching takes less than 40 seconds between the start of the quenching cycle and the first stage of the quenching (resulting in a temperature of the feed surface of 600 ℃), during which time an oxide layer is formed, thus forming a decarburising barrier on the feed. In particular the arm manipulator, so that the transfer time between the oven and the chamber does not exceed 30 seconds.

The parts that have been subjected to the quenching process of the present invention are inspected. Metallurgical studies carried out on gears and shafts carbonitrided and air-quenched under the above conditions show that:

the microhardness distribution achieved for the tooth flanks and tooth roots is identical to that obtained after direct quenching in an oil at 870 ℃. The graph of fig. 3 shows microhardness values in terms of vickers hardness, measured at different depths of the treated part, i.e. vickers hardness values measured at different depths of the treated part for the pressurized air quench on the one hand and vickers hardness values measured at different depths of the treated part for the oil quench on the other hand, the measured values being almost identical.

Pressurized air quenching of parts without decarburization. FIG. 2 shows the distribution of carbon concentration on the tooth flanks and tooth roots of a gear wheel. The carbon concentration profile is equivalent between oil quenching and pressurized air quenching. They are characterized by the absence of surface decarburization of the quenched carburized layer. The first measurement was associated with a part that had been subjected to carbonitriding treatment, followed by a pressurized air quench at 870 ℃. The second batch of measurements was related to the parts that had undergone carbonitriding treatment with oil quenching.

The pressurized air quenching results in the formation of an oxide layer with a thickness of 6 μm.

The invention also includes a method in which the transfer is carried out under a protective atmosphere and the quenching thereof is carried out under pressurized air at a pressure above atmospheric pressure. Under these conditions, in the first stage of quenching, decarburization at the level of quenching is essentially prevented due to the formation of an oxide layer.

The invention also relates to a method wherein the quenching fluid is air and a gas is added to the air to improve its density and/or thermal conductivity.

The invention also relates to a method wherein the quenching fluid is air and a sprayed liquid is added to the air, whereby the part can be cooled by a two-phase mixture.

Claims

1. A method of quenching a steel feedstock after carburizing or carbonitriding treatment at atmospheric pressure, comprising the steps of:

a) extracting the feed from a treatment furnace (1) at a temperature between 750 ℃ and 1100 ℃,

b) transferring the feed material to a quenching chamber (3),

c) injecting quenching fluid with pressure greater than atmospheric pressure, circulating the quenching fluid in the quenching chamber,

d) the feed is cooled to a temperature below 400 c,

the method is characterized in that: the fluid, which is mostly composed of air, causes the temperature of the steel part to be at most 400 ℃ during the time it leaves the treatment furnace (1), and during this time an oxide layer is formed which prevents decarburization of the steel.

2. A process according to claim 1, characterized in that the transfer is effected in the atmosphere and the feed temperature is reduced to 600 ℃, preferably not more than 20 seconds, in less than 40 seconds.

3. A method according to claim 1 or 2, characterized in that the oxide layer is formed to a thickness of less than 12 μm, preferably less than 5 μm.

4. A method according to claim 1, characterized in that the transfer is effected under a protective atmosphere.

5. A method according to any one of claims 1 to 4, wherein the fluid is a mixture of air and a gas which alters the density and/or thermal conductivity of the fluid.

6. A method according to any one of claims 1 to 4, characterised in that the fluid is an air/spray liquid two-phase mixture.

7. Method according to any one of the preceding claims, wherein the quenching fluid inside the quenching chamber is at a pressure between 3 bar and 40 bar, preferably between 5 bar and 20 bar.

8. Apparatus for carrying out the method according to one of the preceding claims, characterized in that it comprises a transfer manipulator and a quenching chamber, which allow the feed between the treatment furnace and the quenching chamber to be maintained at a surface temperature of 600 ℃ for a period of not more than 40 seconds.

9. Apparatus according to the preceding claim, characterised in that the manipulator allows the transfer to take place in a time not exceeding 30 seconds.

10. An apparatus according to claim 8 or 9, characterized in that the quenching chamber comprises a turbine which allows the surface temperature of the fed parts to be reduced to 400 ℃ in a time not exceeding 10 seconds.

11. An apparatus according to any one of claims 8 to 10, characterized in that it is associated with continuous, batch, bell and shaft furnaces operating at atmospheric pressure.