EP2813584A1

EP2813584A1 - System and method for quenching a heated metallic object

Info

Publication number: EP2813584A1
Application number: EP13002983.8A
Authority: EP
Inventors: Akin Malas
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2013-06-11
Filing date: 2013-06-11
Publication date: 2014-12-17

Abstract

The present invention relates to a system and a method for quenching a heated metallic object. The system comprises
- a detector (7) for detecting the dimension of a metallic object (2),
- a furnace (3) for heating the metallic object (2),
- a quenching device (4) comprising at least one nozzle (5) for impinging a fluid (6) onto the heated metallic object (2),
- a central control unit (8) connected to the detector (7) and to the quenching device (4), wherein the central control unit (8) is embodied in such a way that the intensity of quenching is determined according to the detected dimension of said metallic object (2).

Description

The present invention relates to a system and a method for quenching a heated metallic object.
US 6,554,926 B2 discloses a method for quenching a heated metallic object, wherein a plurality of discrete gas streams from a plurality of nozzle outlets is discharged so that the gas streams impinge substantially uniformly onto the outer surface of the object. The distance between each nozzle outlet and the outer surface of the object against which the associated gas stream impinges is less than or equal to half the diameter of the nozzle outlet. Due to very small distance between the gas stream nozzle outlet and the surface of the object, areas of high turbulence are produced at the edges of the nozzles, which interact with the surface of the object to maximize the transfer of heat to the gas and to produce more uniform cooling.
High heat transfer rates are necessary for achieving an effective quenching. Quenching means rapidly chilling a metallic object from a heat treatment temperature in the austenitic range to a much lower, usually room temperature. Quenching can significantly improve the mechanical properties and characteristics of a metallic object. Quenching is used to harden the object and/or to improve its mechanical properties, by controlling internal crystallization and/or precipitation.
Traditionally, quenching has been carried out by using liquids such as water, oil, or brine, either in the form of an immersion bath or a spraying system. In more recent years, gas quenching methods have been developed using multiple cooling gas streams comprising mainly nitrogen, argon and/or helium at pressures up to 60 bar.
An important quenching application is the sinter hardening process. Sinter hardening may be described as hardening of a sintered compact by providing a sufficient cooling rate in a cooling zone of a continuous furnace or in a cooling cycle of a badge type furnace. It therefore combines sintering and hardening in a single furnace to produce the required microstructures in a one step process. The sinter hardening process is becoming a key process in the powder metal manufacturing industry due to pressures to reduce cost. It is possible to eliminate multiple processes by combining them into a single lower cost alternative process that delivers the same metallurgical and mechanical properties.
Christoph Laumen et al., "advanced carbon control in sintering atmospheres", proceedings EURO PM 2009 Copenhagen, Vol. 3, page 233-238 relates to the controlling of a sintering atmosphere, that means of an oxygen probe. By controlling a carbon neutral or carburizing atmosphere, a surface decarburization can be avoided giving higher quality parts and increasing product performance.
Akin Malas et al., "A New Approach to Sintering Furnace Atmosphere Control and Sinter Hardening by Gas Impingement Cooling", Advances in Powder Metallurgy & Particulate Materials - 2008; Proceedings of the 2008 World Congress on Powder Metallurgy & Particulate Materials, June 8-12, Washington, D.C.; ISBN-10: 0-9793488-9-7, part 5 - Pre-Sintering & Sintering describes a newly developed technology for carbon control and flexible cooling systems that can achieve the challenging hardness requirements in a sintering furnace. By means of carbon control and gas quenching, an enhanced control of the sinter hardening process is said to be achieved.
DE 10 2009 040 081 A1 and WO 2011/060769 A1 relate to a laser scanner comprising a light source, a detector array and a lens for imaging a light beam reflected from an object being tested onto the detector array. The light beam is shifted on the detector array according to the distance of the object. Thus the distance of the object can be measured by determining the location of the light beam on the detector array.
An object of the present invention is to provide a system and a method for quenching a heated metallic object that allow to improve the accuracy and repeatability of the sinter hardening process.
This object is solved by a system and a method according to the independent claims. Further improvements of the invention are defined in the corresponding sub-claims.
The system according to the present invention for quenching a heated metallic object comprises

a detector for detecting the dimension of a metallic object,
a furnace for heating the metallic object,
a quenching device comprising at least one nozzle for impinging a fluid onto the heated metallic object,
a central control unit connected to the detector and to the quenching device, wherein the central control unit is embodied in such a way that the desired intensity of impinging fluid on a certain heated metallic object is determined according to the detected dimension of said metallic object.

According to the present invention, firstly the dimension of the metallic object is detected and then, after heating the metallic object, the heated metallic object is quenched, wherein the intensity of quenching is adjusted in accordance with the detected dimension of the metallic object. The intensity of quenching may be adjusted by defining a certain amount of quenching fluid being impinged onto the metallic object, by defining a certain stream of quenching fluid being impinged onto the metallic object, and/or by impinging the quenching fluid for a certain duration onto the object. As the intensity of quenching is adjusted according to the dimension of the heated metallic object, the quenching process is very accurate and repeatable. This makes the complete process easy to control and the complete process is very stable and reliable.
The detected dimension of the metallic object can be a size such as the width, the height, the volume of the metallic object or the weight of the metallic body or a combination thereof. The volume can also be converted into the weight of the metallic body, wherein further information regarding the type of material of which the metallic body is made can be considered. From this data, the intensity of the quenching, particularly the amount of fluid impinged on the heated metallic object, can be precisely determined.
This system allows to process metallic objects of different sizes consecutively, wherein all metallic objects are successfully quenched so that the desired or required microstructures and/or degrees of hardness, strength and/or toughness are achieved while residual stress, distortion, and the risk of cracking is minimized.
The system comprises preferably several quenching zones, wherein in each quenching zone the cooling intensity can be controlled individually. With such a system it is possible that the individual metallic objects undergo different cooling profiles to achieve different physical characteristics.
The system can further comprise an ordinary cooling zone which follows the one or more quenching zones.
The system comprises preferably a conveyer so that a plurality of metallic objects can be continuously and consecutively moved alongside the detector, through the furnace and alongside the quenching device. Thus the present system is suitable for a continuous production of quenched metallic objects. The present invention allows the integration of a quenching step in a regular production environment.
The detector for detecting the dimension of the metallic object can be one or more cameras, a height detection device and/or a scale. Preferably two cameras are used for a top view and a side view so that a three dimensional shape of the body can be deduced from the pictures taken by the cameras. Instead of a camera, it is also possible to use a height detection device, particularly a height detection device for detecting several height points along a certain line so that these height points can be combined into a three dimensional profile. Such height detection devices are known e.g. from DE 10 2009 040 081 A1 and WO 2011/060769 A1 . DE 10 2009 040 081 A1 and WO 2011/060769 A1 are incorporated herein by reference.
Furthermore, a 3D-camera such a TOF-camera can be used for detecting a three dimensional profile of the metallic object. One TOF camera is known from EP 2 594 959 A1 . This document is incorporated herein by reference.
If the camera is provided in combination with a conveyor, a line scan camera is preferred, wherein the scanned line is arranged perpendicular to the transporting direction of the conveyor.
The quenching device can comprise an actuator for adjusting the distance between an outlet of the nozzle and an outer surface of the heated metallic object, wherein this distance is controlled by the central control unit according to the detected dimension of the heated metallic object. This distance lies preferably in the range of 1 mm - 10 mm and preferably 1 mm - 5 mm. If the system comprises several quenching zones, then it is appropriate if in each quenching zone at least one actuator for adjusting the distance between the outlet of the nozzle and the outer surface of the heated metallic object is provided. These actuators are individually controlled, so that in the different quenching zones metallic objects of different sizes can be cooled.
The quenching device or the plurality of quenching devices comprise preferably each a plurality of nozzles. An actuator can be provided for each nozzle so that all nozzles can be adjusted individually.
The furnace is preferably embodied to heat the metallic objects to a temperature in the range of 800°C to 1500°C. The exact temperature depends on the kind of material of the metallic object, wherein the temperature of the metallic object in the furnace should be in the austenitic range.
The quenching fluid can be a gas or a liquid. Suitable gases are nitrogen, argon, helium, carbon monoxide, hydrogen, or a mixture thereof, wherein further components, such as propane, can be added. Suitable liquids are water, oil or brine.
The furnace can be connected to a gas source for providing a certain gas atmosphere. The gas atmosphere is particularly adjusted to control the carbon content for avoiding decarburization particularly in a sintering process.
The furnace can be provided with an oxygen probe for detecting the oxygen content of the atmosphere in the furnace.
The method according to the invention for quenching a heated metallic object comprises the following steps of

detecting the dimension of a metallic object,
heating said metallic object,
quenching the heated metallic object by impinging a fluid onto the heated metallic object,
controlling the intensity of impinging fluid on said heated metallic object according to the detected dimension of said metallic object.

The method according to the present invention is particularly used for sinter hardening. Sintered metallic objects are very sensitive to the quenching step. Moreover, sintered metallic objects can be produced in different shapes and from different kind of metallic powders. The present invention allows to control the quenching step individually for each metallic object in dependence on the shape and/or the metallic material. Thus the present invention allows the continuous production of different types of sintered products, so that use can be made of the complete variety of sintered products.
Therefore, the method according to the present invention provides a highly flexible production of metallic sintered objects with high quality, wherein the production process is highly stable, reliable and repeatable.
The invention will now be described by way of example with reference to the accompanying drawing, in which:

Figure 1: schematically illustrates a system for quenching a heated metallic object in a block diagram.

The system 1 according to the present invention for quenching a heated metallic object 2 comprises a furnace 3 for heating of a metallic object 2, a quenching device 4 having at least one nozzle 5 for impinging a fluid 6 onto the heated metallic object 2, a detector 7 for detecting a dimension of the metallic object 2, and a central control unit 8 connected to the detector 7 and the quenching device 4, wherein the central control unit 8 is embodied in such a way that the intensity of fluid 6 impinging on certain heated metallic object 2 is determined according to the detected dimension of said metallic object 2. A conveyor 9 is provided for moving a plurality of the metallic objects 2 in a transporting direction 10, wherein the metallic objects 2 pass by the detector 7 and through the furnace 3, at least one quenching zone 11 and a cooling zone 12.
The furnace 3 is embodied for heating the metallic object 2 to a temperature in the range of 800°C to 1500°C. The furnace 3 is connected to one or more gas reservoirs 13. Figure 1 shows only one gas reservoir 13. The gas reservoir 13 contains a certain gas, such as carbon monoxide, hydrogen, nitrogen, or a mixture thereof. Further components such as methane and/or propane can be added. A valve 14 is provided for controlling the gas stream from the gas reservoir 13 into the furnace 3. The valve 14 is connected to and controlled by the central control unit 8. If several gas reservoirs are provided then the gas stream of each gas reservoir can be individually controlled, so that the composition of the gas atmosphere in the furnace 3 can be individually changed and adapted to the current thermal treating of the metallic objects 2.
The embodiment according to Figure 1 shows four quenching zones 11 that are following to the furnace 3 in the transporting direction 10. Each quenching zone 11 comprises a quenching device 4 having at least one nozzle 5 and an actuator 15 for adjusting the height of the nozzle 5. The system according to the present invention can comprise any number of quenching zones. If several quenching zones are provided then it is possible to simultaneously treat different objects with different quenching intensities and/or treat one object sequentially with different quenching intensities. Thus the number of quenching zones is determined according to the variety of quenching treatments that are to be carried out and the throughput of the system.
Each nozzle 5 is connected to a fluid reservoir 16 that contains fluid for impinging the heated object 2 therewith. The connecting lines between the fluid reservoir 16 and the nozzles 5 comprise valves 17 for controlling the fluid stream to each nozzle 5. The valves 17 are connected to and controlled by the central control unit 8 so that the fluid stream to each quenching zone 11 can be individually adjusted.
The actuators 15 are connected to and controlled by the central control unit 8 so that the height of the nozzles 5 of each quenching zone 11 and thus the distance between an outlet of the respective nozzles 5 and the outer surface of the metallic objects 2 can be individually adjusted.
The fluid reservoir 16 can contain a gas or a liquid. Suitable gases are carbon monoxide, hydrogen, nitrogen, argon and/or helium or a mixture thereof. Suitable liquids are water, oil and/or brine. If the quenching fluid is a liquid, then a pump is provided for pumping the liquid to the nozzles 5. If the quenching fluid is a gas, then the gas is kept under a certain pressure (e.g. 40 - 80 bar) in the fluid reservoir 16, so that just by opening and closing the valve 17 the gas stream through the nozzles 5 can be exactly controlled.
The cooling zone 12 is a cooling zone for slowly cooling down the quenched objects 2 to room temperature. In the present embodiment the cooling zone 12 is connected by means of an outlet pipe 18 and an inlet pipe 19 to a heat exchanger 20 for cooling the gas atmosphere in the heating zone.
The conveyer 9 comprises preferably a metal mesh belt. Such metal mesh belts can resist the high temperatures in the furnace 3. The mesh belts have a large heat capacity which supports the heat transfer and increases the heating rates in the furnace and the cooling rates in the quenching zones.
The detector 7 can comprise at least one camera for detecting the shape of the metallic objects 2 to be heated. The camera is arranged for monitoring the objects 2 in a top view, side view or in a tilted, perspective view.
Preferably, the detector 7 comprises two or more cameras, one for monitoring the metallic objects 2 in a top view and at least another one for monitoring the metallic objects 2 in a side view.
When a conveyer 9 is provided for moving the objects 2 in a transporting direction 10, then the cameras are preferably line cameras, wherein each camera is adjusted with its detection line perpendicular to the transporting direction 10.
Alternatively it is possible to use a height detection device for detecting the outer surface of the metallic objects. Such height detection devices are known e.g. from DE 10 2009 040 081 A1 and WO 2011/060769 A1 . Such height detection devices can be used for detecting the top surface of the metallic objects if they are arranged above the metallic objects, but also for detecting the side surfaces of the metallic objects if they are arranged laterally of the metallic objects or the conveyor, respectively.
Furthermore, a 3D-camera, such as a TOF-camera, can be used for detecting a three-dimensional profile of the metallic objects.
Instead of the optical measuring devices (camera, height detection device) or additionally the detector can comprise a scale for detecting the weight of each object 2.
The detector 7 is connected to the central control unit 8. The central control unit 8 is embodied for analyzing the signals, particularly the pictures, provided by the detector 7 for extracting a dimension of the metallic object 2. The dimension can be the height and/or the width and/or the length and/or the volume and/or the weight of the metallic body. For determining the weight of the metallic body only with an optical measuring device the specific weight of the type of material of which the metallic body is made from has to be considered. If the optical measuring device is a color camera, then it is possible to conclude from the detected color the kind of material of which the metallic object is made, for automatically determining the specific weight of this material.
Preferably a combination of a scale and an optical measuring device (camera, height detection device) is provided, so that conclusions of the kind of material can be drawn from the weight and the size of the metallic object. These conclusions can cover the porosity of the metallic object which can be an important parameter for adjusting the quenching intensity.
According to the present invention for each metallic object to be heated at least one dimension (height, width, length, form, volume, weight) is detected and on the basis of this/these dimension(s) the quenching intensity is controlled by adjusting the amount of quenching fluid supplied and/or the stream of the quenching fluid, and/or the distance between the metallic object and the corresponding nozzle 5 and/or the duration of impinging the fluid 6 onto the object 2. The larger or the heavier the metallic object 2 is, the stronger the quenching intensity should be. However, the quenching intensity also depends on the desired characteristics of the final product, particularly with respect to the required microstructure, hardness, strain, toughness, residual stress, distortion, and the risk of cracking. In dependence of the desired characteristics of a final product, a certain cooling rate should be adjusted. As at least one dimension of the metallic object is detected, the desired cooling rate can be exactly controlled according to the detected dimension. Typical cooling rates of the quenching process lie in the range of 1 - 6 C°/s.
The above-described system can be used for sintering metallic objects. Such a metallic object 2 is made of a metallic powder, particularly made of steel powder containing one or more alloying element, such as chromium (Cr), molybdenum (Mo), nickel (Ni), manganese (Mn), copper (Cu), or carbon (C). The metallic object 2 is heated in the furnace 3 to a temperature in the range of about 800°C - 1500°C, wherein the metallic object is heated to the austenitizing or solution treating temperature, which lies typically in the range of 815°C - 870°C for steel. The sintering can be carried out in vacuum or under a protective atmosphere. If a protective atmosphere is used then the alloying elements play the key role in determining the atmosphere composition. Preferably a constant mixture of CO, H₂ and N₂ and small amounts of CO₂ are used. The most commonly used sintering atmosphere types are N₂+10% H₂ or endogas or mixtures of these gases. Sometimes methane or propane is added for carbon control. A conventional endogas sintering atmosphere contains water (about 0.7%) and is rich in CO and H₂.
Due to the thermal treatment in the furnace 3, the metallic objects are formed to sintered compacts.
The sintered compacts are quenched in the quenching zones 11, wherein the quenching intensity is adjusted according to a detected dimension of each metallic object 2. In the present embodiment the objects 2 are detected by two cameras in a top view and a side view, so that the length, height and width of each object 2 is determined so that the volume and the weight of each object can then be calculated/estimated. The cooling rates in the quenching zone are adjusted according to the determined volume/weight. These cooling rates lie in the range of 1 - 6°C/s. The distance between the outer surface of the metallic objects 2 and the nozzles 5 is adjusted in a range of 1 mm - 10 mm and preferably 1 mm - 5 mm.
The quenched metallic object 2 is then cooled down to room temperature in the cooling zone 12.
According to the above described embodiment the method for quenching a heated metallic object is used to improve sinter hardening. However, the method for quenching a heated metallic object according to the present invention can also be used to improve the quenching step of any other heat treatment, particularly, any heat treatment using mesh conveyer belts in the furnace as well as in the quenching zone. Furnace brazing is an example of such a heat treatment comprising a quenching step.
The invention is described above by means of an embodiment for continuously heating, quenching and cooling a series of metallic objects. This is a continuous process. The present invention can be also adapted for a batch process, wherein a certain numbers of objects are simultaneously heated, then quenched and cooled to room temperature. The quenching can be carried out by spraying a fluid onto the objects as it is done in the above described embodiment. Alternatively the objects can also be quenched by immersing them into a bath which contains a quenching fluid.

List of references

1: System
2: Metallic object
3: Furnace
4: Quenching device
5: Nozzle
6: Fluid
7: Detector
8: Central control unit
9: Conveyor
10: Transporting direction
11: Quenching zone
12: Cooling zone
13: Gas reservoir
14: Valve
15: Actuator
16: Fluid reservoir
17: Valve
18: Outlet pipe
19: Inlet pipe
20: Heat exchanger

Claims

System for quenching a heated metallic object, comprising
- a detector (7) for detecting the dimension of a metallic object (2),

- a furnace (3) for heating the metallic object (2),

- a quenching device (4) comprising at least one nozzle (5) for impinging a fluid (6) onto the heated metallic object (2),

- a central control unit (8) connected to the detector (7) and to the quenching device (4), wherein the central control unit (8) is embodied in such a way that the intensity of impinging fluid (6) on a certain heated metallic object (2) is determined according to the detected dimension of said metallic object (2).
System according to claim 1, further comprising a conveyer (9), particularly a mesh belt conveyor for moving a plurality of metallic objects (2) continuously and consecutively along the detector (7), through the furnace (3) and along the quenching device (4).
System according to claim 1 or 2, wherein the detector (7) comprises a camera, a height detection device and/or a scale.
System according to any one of the claims 1 to 3, wherein the quenching device (4) comprises an actuator (15) for adjusting the distance between an outlet of the nozzle (5) and an outer surface of the heated metallic object (2), wherein this distance is controlled by the central control unit (8) according to the detected dimension of the heated metallic object (2).
System according to any one of the claims 1 to 4, wherein the quenching device (4) comprises a plurality of nozzles (5).
System according to any one of the claims 1 to 5, wherein the furnace (3) is embodied for heating the metallic object (2) to a temperature in the range of 800°C to 1500°C and particularly from 800°C to 1200°C.
System according to any one of the claims 1 to 6, wherein the furnace (3) is connected to a gas reservoir (13) for providing a certain gas atmosphere, wherein in particular the gas atmosphere comprises nitrogen, carbon monoxide, hydrogen, or a mixture thereof, wherein further components, such as methane or propane, can be added.
System according to claim 7, wherein the furnace (3) comprises an oxygen probe for detecting the oxygen content of the atmosphere in the furnace.
Method for quenching a heated metallic object, comprising the following steps of
- detecting a dimension of a metallic object (2),

- heating said metallic object (2),

- quenching the heated metallic object (2) by impinging a fluid (6) onto the heated metallic object (2),

- controlling the quenching intensity according to the detected dimension of said metallic object (2).
Method according to claim 9, wherein the intensity of quenching is adjusted by either impinging a certain amount of quenching fluid (6) onto the metallic object (2) and/or by adjusting the stream of quenching fluid (6) being impinged onto the metallic object (2) and/or by adjusting the duration of impinging the quenching fluid (6) onto the object and/or by adjusting the distance between an outer surface of the metallic object (2) and an outlet of a nozzle (5) for applying the fluid (6).
Method according to claim 9 or 10, wherein the detected dimension of the metallic object (2) is the width, the height, the volume or the weight of the metallic body (2).
Method according to any one of the claims 9 to 11, wherein a gas comprising CO, hydrogen, nitrogen, helium, argon or a mixture thereof or a liquid is used as fluid for impinging the heated metallic object.
Method according any one of the claims 9 to 12, wherein the metallic object comprises metallic powder which is sintered by said heating of the metallic object.
Method according to any one of the claims 9 to 13, wherein a plurality of metallic objects are continuously and consecutively heated and quenched.
Method according to any one of the claims 9 to 14, wherein a system according to any one of the claims 1 to 8 is used.1.