US20140353887A1

US20140353887A1 - Metallurgical heat treating system with heat recovery

Info

Publication number: US20140353887A1
Application number: US14/463,796
Authority: US
Inventors: Jonathon R. Crow; Michael Pershing
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2014-08-20
Filing date: 2014-08-20
Publication date: 2014-12-04

Abstract

A metallurgical heat treating system with heat recovery is disclosed herein. The metallurgical heat treating system includes a conveyor, a first furnace, a second furnace, a cooling section, a preheating section, and a heat recovery duct. The conveyor conveys material in a conveying direction. The first furnace, the second furnace, the cooling section, and the preheating section are in thermal communication with the conveyor. The second furnace is disposed downstream of the first furnace in the conveying direction. The cooling section is disposed downstream of the second furnace in the conveying direction. The preheating section is disposed upstream of the first furnace in the conveying direction. The heat recovery duct is in fluid communication with the cooling section and the preheating section for a convective transfer of heat therebetween.

Description

TECHNICAL FIELD

The present disclosure relates generally to metallurgical heat treating systems. More specifically, the present disclosure relates to a metallurgical heat treating system with heat recovery.

BACKGROUND

Iron alloys are often made to undergo various heat treatment processes to change the mechanical properties of the iron alloys. One such process may be a hardening and tempering process that improves hardness and toughness of a carbon containing iron alloy. In this process, a workpiece of iron alloy is made to pass through a hardening furnace, followed by a tempering furnace.
In the hardening furnace, the workpiece is heated to relatively high temperatures and then suddenly cooled to impart hardness to the workpiece. After being hardened, the workpiece is passed through the tempering furnace.
In the tempering furnace, the workpiece is again heated to a high temperature to impart toughness to the workpiece. After the parts have been heated and soaked at the proper temperature, the parts are discharged from the tempering furnace while still at essentially the tempering temperature. For safety and ergonomic reasons, the residual heat is often removed from the parts and furnace fixturing with a cool-out section. The cool-out section pushes a blast of lower temperature air towards the workpiece, to cool down the workpiece. The blast of air convectively absorbs the heat from the parts as the air passes over any loads in the cool-out section. Typically, this heated air is released into the environment, resulting in a significant waste of thermal energy.
U.S. Pat. No. 8,298,475 discloses a furnace system having a preheating zone, a high temperature heating zone, two cooling zones, and a piping system. In the preheating section, high chromium steel parts are initially heated to a certain temperature before entering the high temperature heating zone, to avoid distortion or twist of the parts. The piping system extracts heat from waste gases of distinct cooling zones, and delivers the extracted heat to the preheating zone and/or the high temperature heating zone. However, the heat extracted from the waste gases alone is insufficient to meet heating requirements of the preheating zone, and therefore external heating sources are installed in the preheating zone to fulfil the heating requirements. Moreover, the waste gases delivered to the preheating zone and/or the high temperature heating zone is controlled based on oxygen concentrations in these zones. Heat recovery efficiency will vary based on oxygen control system needs, and may be reduced to near zero in cases where the control system must stop preheating. Although, this reference provides the furnace system with heat recovery from the waste gases, the configuration shown requires complex oxygen sensing and associated damper control to maintain the integrity of the atmosphere in the high heat section.

SUMMARY OF THE INVENTION

Various aspects of the present disclosure are directed to a metallurgical heat treating system with heat recovery. The metallurgical heat treating system includes a conveyor with a preheating section, a first furnace, a second furnace, a cooling section, and a heat recovery duct. The conveyor is configured to convey material in a conveying direction. The first furnace is in thermal communication with the conveyor. The second furnace is in thermal communication with the conveyor, and is disposed downstream of the first furnace in the conveying direction. The cooling section is in thermal communication with the conveyor, and is disposed downstream of the second furnace in the conveying direction. The preheating section is in thermal communication with the conveyor, and is disposed upstream of the first furnace in the conveying direction. The heat recovery duct is in fluid communication with the cooling section and the preheating section for a convective transfer of heat therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top view of a plant layout that employs a single metallurgical heat treatment system along with a heat recovery system to improve the mechanical properties of a steel workpiece, in accordance with the concepts of the present disclosure; and

FIG. 2 is a schematic top view of the plant layout that employs two metallurgical heat treatment systems along with a heat recovery system, each of which is similar to the single metallurgical heat treatment system of FIG. 1.

DETAILED DESCRIPTION

Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Wherever possible, corresponding or similar reference numbers will be used throughout the drawings to refer to the same or corresponding parts. Referring to FIG. 1, there is shown a schematic top view of a plant layout that employs a single metallurgical heat treating system 100 to improve the mechanical properties of a workpiece. The metallurgical heat treating system 100 may embody either of a hardening system, a tempering system, an annealing system, a precipitation strengthening system, and/or a combination of two or more thereof. The present disclosure contemplates a combination of a hardening and tempering process performed on a steel workpiece to improve the hardness and toughness of the steel workpiece. The metallurgical heat treating system 100 includes a loading station 102, a conveyor 104, a first furnace 106, a quench bath section 108, a second furnace 110, a cooling section 112, a preheating section 114, an unloading station 116, and a heat recovery duct 118.
The loading station 102 is an area where raw steel workpieces are kept in the form of sheets, plates, blocks, castings, fabrications, or machined parts. The loading station 102 may employ an automatic machine, a semi-automatic machine, and/or a manual-feed mechanism to place the workpiece onto the conveyor 104. For example, the loading station 102 may employ a robotic arm to pick up and place the workpiece onto the conveyor 104.
The conveyor 104 may be a belt conveyor, a roller conveyor, and/or a band conveyor. The conveyor 104 runs from the loading station 102 to the unloading station 116, through the preheating section 114, the first furnace 106, the quench bath section 108, the second furnace 110, and the cooling section 112. The conveyor 104 supports and conveys the workpiece in a conveying direction 120, while transporting the workpiece through the preheating section 114, the first furnace 106, the quench bath section 108, the second furnace 110, and the cooling section 112.
The first furnace 106 may be a hardening furnace adapted to heat the workpiece to an austenite temperature (800° C.-1000° C.), as it passes through the first furnace 106. The first furnace 106 is structured and arranged at a portion of the conveyor 104 to be in thermal communication with the conveyor 104. In an embodiment, the first furnace 106 may be one or more infrared heating chambers disposed around the conveyor 104 that emit infrared rays to heat the workpiece. Alternatively, the first furnace 106 may be a support structure that employs heating tubes to heat the workpiece. It may be understood that the first furnace 106 may embody any conventional structural arrangement that may heat the workpiece to the austenite temperature, as it passes through the first furnace 106.
The quench bath section 108 may be a cooling region adapted to rapidly cool the workpiece passing therethrough to a low temperature range (30° C.-40° C.). The quench bath section 108 is disposed at another portion of the conveyor 104 disposed downstream of the first furnace 106 in the conveying direction 120. The quench bath section 108 may be a pool of water, coolant, polymer, and/or similar material that cools the workpiece passing therethrough.
The second furnace 110 may be a tempering furnace employed to re-heat the workpiece to a relatively lesser temperature (150° C.-700° C.) as compared to the first furnace 106. The second furnace 110 is structured and arranged at yet another portion of the conveyor 104, disposed downstream of the first furnace 106, beyond the quench bath section 108, in the conveying direction 120. The second furnace 110 is in thermal communication with the conveyor 104, and is similar in construction to that of the first furnace 106.
The cooling section 112 is a section wherein the workpieces are gradually cooled to a temperature lesser than 60° C. to facilitate safe and ergonomic handling of the workpiece. The cooling section 112 is structured and arranged at yet another portion of the conveyor 104, disposed downstream of the second furnace 110. The cooling section 112 is in thermal communication with the conveyor 104, and is adapted to cool the workpiece as it passes through the cooling section 112. Typically, the cooling section 112 employs a cooling fan 122, which generates a blast of air and/or cooling gases along the workpiece. This enables the cooling gases to convectively extract heat from the workpiece, when passed over the workpiece. Thereafter, the workpiece is sent to the unloading station 116 to be unloaded from the conveyor 104. At this point, the hot cooling gases exit from the cooling section 112.
Hardness and toughness of the workpieces are substantially increased as they are passed through the first furnace 106, the quench bath section 108, the second furnace 110, and the cooling section 112. The workpieces are initially passed through the preheating section 114 to be preheated, thereby extracting otherwise wasted heat and improving the overall energy efficiency of the system.
The preheating section 114 is a section, where workpieces are initially heated to a relatively lesser temperature than the temperature (800° C.-1000° C.) of the first furnace 106, before entering the first furnace 106. The preheating section 114 is structured and arranged at yet another portion of the conveyor 104, disposed upstream of the first furnace 106 in the conveying direction 120. The preheating section 114 is in thermal communication with the conveyor 104, to heat the workpiece as it passes through the preheating section 114. Heat is transferred to the workpieces in the preheating section 114 solely via convective collection of heat in the cooling section 112 and convective delivery into the preheating section 114 via the heat recovery duct 118.
The heat recovery duct 118 is provided to allow a fluid communication between the cooling section 112 and the preheating section 114. More particularly, the heat recovery duct 118 provides fluid communication between the cooling fan 122 of the cooling section 112 and the preheating section 114. The heat recovery duct 118 carries gases from an exhaust of the cooling fan 122 to the preheating section 114. As the gases are heated to a high temperature at the exhaust of the cooling fan 122, they convectively heat the workpiece when introduced to the preheating section 114.
Referring to FIG. 2, there is shown a plant layout with an alternate embodiment of the present disclosure, which includes a first metallurgical heat treating system 200 and a second metallurgical heat treating system 200′. Both of the metallurgical heat treating systems 200 and 200′ are similar in construction to that of the metallurgical heat treating system 100, as explained in FIG. 1. Each of the metallurgical heat treating systems 200 and 200′, include loading stations 202, 202′, conveyors 204, 204′, first furnaces 206, 206′, quench bath sections 208, 208′, second furnaces 210, 210′, cooling sections 212, 212′, preheating sections 214, 214′, and unloading stations 216, 216′. In an embodiment, there is provided a first heat recovery duct 218, a second heat recovery duct 224, a third heat recovery duct 226, and a fourth heat recovery duct 228. The first heat recovery duct 218 provides fluid communication between the cooling section 212 of the first metallurgical heat treating system 200 and the preheating section 214 of the first metallurgical heat treating system 200. The second heat recovery duct 224 provides fluid communication between the cooling section 212 of the first metallurgical heat treating system 200 and the preheating section 214′ of the second metallurgical heat treating system 200′. The third heat recovery duct 226 provides fluid communication between the cooling section 212′ of the second metallurgical heat treating system 200′ and the preheating section 214′ of the second metallurgical heat treating system 200′. The fourth heat recovery duct 228 provides fluid communication between the cooling section 212′ of the second metallurgical heat treating system 200′ and the preheating section 214 of the first metallurgical heat treating system 200.
In operation, the first heat recovery duct 218 supplies gases from the cooling section 212 of the first metallurgical heat treating system 200 to the preheating section 214 of the first metallurgical heat treating system 200. Gasses are used to preheat the workpiece as it passes within the preheating section 214. Whereas, the second heat recovery duct 224 supplies gases from the cooling section 212 of the first metallurgical heat treating system 200 to the preheating section 214′ of the second metallurgical heat treating system 200′. Similarly, the third and fourth heat recovery ducts 226 and 228 supply gases from the cooling section 212′ respectively to the preheating sections 214 and 214′. This enables exchange of heat between the cooling sections 212, 212′ and the preheating sections 214, 214′.

INDUSTRIAL APPLICABILITY

In operation, the steel workpieces to be heat treated are placed onto the conveyor 104 at the loading station 102. The conveyor 104, which moves in the conveying direction 120, transports the workpiece in a sequential manner through the preheating section 114, the first furnace 106, the quench bath section 108, the second furnace 110, and the cooling section 112.
Initially, the preheating section 114 alone convectively heats the workpiece to preheat the workpiece, by recovering waste heat from the cooling section 112. After preheating, the workpiece is passed through the first furnace 106, where the workpiece is heated to a very high temperature (800° C.-1000° C.). The conveyor 104 then carries the workpiece through the quench bath section 108, where the workpiece is suddenly cooled. This imparts hardness to the work-piece. Thereafter, the conveyor 104 carries the workpieces through the second furnace 110, where the workpiece is heated to a relatively less temperature range (150° C.-700° C.) as compared to that of the temperature range (800° C.-1000° C.) of the first furnace 106. Further, the work-piece is transported to the cooling section 112, where workpiece is gradually cooled below 60° C. of temperature, to allow for safe and ergonomic handling, before being unloaded in the unloading station 116.
In the cooling section 112, the cooling fan 122 generates the blast of air through the work-piece to cool the work-piece. The heat recovery duct 118 is provided at the exhaust of the cooling fan 122 that carries the gases to the preheating section 114 to enable convective heat exchange between the cooling section 112 and the preheating section 114.
Moreover, as the metallurgical heat treating system 100 is a continuous process, new workpieces are regularly introduced in the preheating section 114, to undergo the complete cycle of the metallurgical heat treating system 100. The gases from the cooling section 112 may heat up the new workpieces as they pass through the preheating section 114. Hence, minimal control systems are required by the preheating section 114 to pre-heat the new work-piece. Further, the flow of gases from the cooling section 112 to the preheating section 114 eliminates need of sensors, such as an oxygen density sensor, to determine preheating demands. Also, waste gases in the present disclosure may not interact in any way with the function of the high heat sections (the first furnace 106 and the second furnace 110), except to preheat the work-piece for energy recovery. Thus, the waste heat is efficiently utilized, which enables heat recovery from the cooling section 112. This results in effective energy cost saving of the metallurgical heat treating system 100.
Similarly, in the alternate embodiment, the first metallurgical heat treating system 200 and the second metallurgical heat treating system 200′, are used to harden and temper the work-pieces. Workpieces are passed through the preheating sections 214, 214′ the first furnaces 206, 206′ the quench bath sections 208, 208′ the second furnaces 210, 210′ and the cooling sections 212, 212′ of each of the first and second metallurgical heat treating systems 200, 200′, for being hardened and toughened. The first heat recovery duct 218, the second heat recovery duct 224, the third heat recovery duct 226, and the fourth heat recovery duct 228, enable heat recovery from the first and second metallurgical heat treating systems 200, 200′. More particularly, the first and second heat recovery ducts 218, 224 carry gases from the cooling section 212 of the first metallurgical heat treating system 200 respectively to the preheating sections 214, 214′. This enables heat recovery from the cooling section 212 of the first metallurgical heat treating system 200. Whereas, the third and fourth heat recovery ducts 226, 228 carry cooling gases from cooling section 212′ of the second metallurgical heat treating system 200′ respectively to the preheating sections 214′, 214. This enables heat recovery from the cooling section 212′ of the second metallurgical heat treating system 200′. Therefore, heat recovery between various sections of the first and second metallurgical heat treating systems 200, 200′ effectively reduces operational costs.
It should be understood that the above description is intended for illustrative purposes only and is not intended to limit the scope of the present disclosure in any way. Those skilled in the art will appreciate that other aspects of the disclosure may be obtained from a study of the drawings, the disclosure, and the appended claim.

Claims

What is claimed is:

1. A metallurgical heat treating system comprising:

a conveyor configured to convey material in a conveying direction;

a first furnace in thermal communication with the conveyor;

a second furnace in thermal communication with the conveyor, the second furnace being disposed downstream of the first furnace in the conveying direction;

a cooling section in thermal communication with the conveyor, the cooling section being disposed downstream of the second furnace in the conveying direction;

a preheating section in thermal communication with the conveyor, the preheating section being disposed upstream of the first furnace in the conveying direction; and

a heat recovery duct in fluid communication with the cooling section and the preheating section for a convective transfer of heat therebetween.