US3278349A - Method of reducing warpage and dimensional growth of structural members during heat treatment - Google Patents

Description

Oct. 11, 1966 A HUSEBY ETAL 3,278,349

METHOD OF REDUCING WARPAGE AND DIMENSIONAL GROWTH OF STRUCTURAL MEMBERS DURING HEAT TREATMENT Filed May 1, 1964 DIMENSIONAL CHANGE (TEN THOUSANDS OF AN INCH PER INCH) 5. -5 '0 5 I0 I I5 20 E ROBERTJAWHMQJZENTOR DIMENSIONAL CHANGE "13? (TEN THOUSANDS OF AN INCH /mfru; FJ fiarle PER INCH) United States Fatent G 3,278,349 METHOD OF REDUCING WARIAGE AND DIMEN- SIGNAL GROWTH OF STRUCTURAL MEMBERS DURING HEAT TREATMENT Robert A. Huseby and Edmond J. Nielsen, Milwaukee, Wis., assignors to A. 0. Smith Corporation, Milwaukee, Wis, a corporation of New York Filed May 1, 1964, Ser. No. 364,119 5 Claims. (Cl. 148-431) This invention relates to metal processing and more particularly to a method of minimizing warpage and dimensional growth in elongated structural members during heat treatment.

In the past, the quenching and subsequent straightening of long channel members, such as those used as truck trailer side rails, has been almost a completely manual operation. Channel members for truck trailer frames range in length from 25 to 40 feet and are first heated and then quenched and tempered to increase the tensile strength and hardness of the frame. The payload which may be carried under the gross weight limit laws for State and Federal highways is thereby increased because a lighter, stronger frame results. The heat treatment causes considerable warpage, such as twisting, bowling and cambering, and the warpage must be removed before the channel member is acceptable for use as a truck or truck trailer side rail. In the past, the channel members were heated, quenched and tempered and then cold straightened manually with the aid of hydraulic presses, and primitive straightening bars to remove all warpage from the channel member.

More recently, it has been proposed to die-quench the channel members in order to reduce the warpage. In the die-quenching operation, the channel member is taken from the furnace and held in a die during the quenching operation. This die-quenching operation substantially reduces the warpage in the long channel member, and therefore minimizes the amount of manual straightening that is required. While the die-quenching operation eliminates a substantial amount of the warpage, the die cannot hold the long workpiece against all warpage. If a suificient force is exerted to hold the long channel member against all warpage, cracking of the member results. In addition, even when using the die-quenching operation, dimensional growth of the channel is produced and dimensional growth is undesirable.

During the blanking operation, the flat metal blank is not only cut to size, but is also provided with a series of holes which are used for connections to other structural members in the vehicle frame. In some side rails for trucks and truck trailers, the blank may be provided with up to 300 holes. During the heat treating operation, a variation in dimensional growth occurs. This dimensional growth may vary substantially from piece-to-piece so that the holes are not properly aligned when the channel members are assembled into the frame, and in many cases the holes must be re-drilled in order to assemble the frame.

The present invention is directed to a method of minimizing dimensional growth and warpage in elongated structural members during heat treatment. According to the invention, the elongated structure member, such as a channel or angle, is initially heated to -a temperature above the upper critical temperature, and .then subjected to a two-step cooling treatment. The member is initially cooled from a temperature above the upper critical temperature to a temperature below the upper critical temperature in the range of 1300 F. to 1650 F. and is then subjected to a die-quenching treatment which reduces the temperature to below 800 F.

Patented Oct. 11, 1966 ICC This two-step cooling treatment for the elongated structural member minimizes warpage, including bowing, cambering and twisting and thereby substantially eliminates the expensive hand-straightening operation which was necessary with prior heat treating methods. In addition, the treatment of the invention reduces the dimensional growth of the member and thus makes assembly of the frame easier and eliminates the re-drilling of holes.

Other objects and advantages will appear in the course of the following description.

In the drawings:

FIG. 1 is a normal frequency curve illustrating the variation in dimensional growth of carbon steel channels subjected to a single step die-quenching operation; and

FIG. 2 is a curve similar to FIG. 1 showing the reduced dimensional growth in a carbon steel channel subjected to the two-step die-quenching operation of the invention.

The structural member which is to be subjected to the heat treatment of the invention is generally a channel or angle having a thickness less than /2 inch and having a substantial length usually in the neighborhood of 20 to 40 feet. The invention is particularly adapted to the heat treatment of carbon steel truck side rails which are channel shaped in cross section, having a length of 25 to 40 feet, and a thickness of A inch to inch.

The steel structural member to be treated has a total alloy content less than 3% by weight and has a carbon content less than 0.30% and generally in the range of 0.15 to 0.30%. In addition, the member may contain 0.85 to 1.50% manganese. Phosphorous should be maintained less than 0.04%, sulfur should be maintained below 0.05% The steel structural member is initially heated to a temperature above the upper critical temperature and generally to a temperature in the range of 25 to 50 above the upper critical temperature. For most alloys, this temperature will be in the range of 1500 F. to 1900 F. Heating to this temperature can be at any desired rate and the rate of heating is not critical The member is held at this temperature for a period of time sufficient to obtain a uniform temperature and carbon distribution throughout the piece. Generally, the member will be held at the temperature for a period of time of 2 to 15 minutes.

Following the holding temperature, the member is cooled to a temperature below the upper critical temperature and generally to a temperature in the range of 1300 to 1650 F. The cooling rate to the temperature of 1300 F. to 1650 F. is not critical and the member can be cooled to this temperature range by furnace cooling, atmosphere cooling or by quenching in a salt bath or the like. Cooling to the range of 1300 F. to 1650 F. should be accomplished in less than 30 minutes for most articles and usually less than 15 minutes.

Members having a carbon content in the lower portion of the aforementioned carbon range are usually cooled to a temperature in the upper portion of the l300 to 1650 F. range, while conversely, members having a carbon content in the upper portion of the aforementioned carbon range are usually cooled to a temperature in the lower portion of the l300 to 1650" F. range.

The member is held at the temperature of 1300 to 1650 F. for a period of time sufficient to get a uniform distribution of temperature. This period is generally in the range of about 3 to 15 minutes.

The member is then inserted into a die and quenched to a temperature below 800 F. at a rate faster than 400 F. per second, and usually in the range of 400 F. to 1600 F. per second, by using oil, water or the like. The member can be die-quenched with the apparatus disclosed in the copending application of Gerald J. Kuchera, Serial No. 73,954, filed December 5, 1960 and entitled Method and Apparatus for Quenching a Metal Member. As disclosed in this prior application, the die-quenching apparatus comprises expandable male and female dies and the channel member is inserted between the dies. Both sets of dies have a network of grooves which gives the die surfaces a waflle-like appearance, and the dies have internal passages which communicate with the wattle-like surfaces. A cooling medium, such as water, is supplied through the passages to the wattle-like surfaces of both the male and female die members which are disposed in contact with the channel member.

In operation, the heated channel member is transferred web down into the female dies of the die sections. The movable side wall of the female die is then moved in, clamping the hot channel member between fixed and moveable side walls of the female die sections thereby straightening the channel member. The expandable male dies are then moved toward the female die members and into the channel member throughout its length and extended laterally into engagement with the side flanges of the channel member. Quenching water is then circulated under pressure through the cooling system and the cooling water flows abundantly through the network of intersecting grooves in the male and female die surfaces and into direct contact with all surfaces of the hot channel member. The cooling water quenches the channel member rapidly and uniformly while it is being held by the die assemblies. The male dies are then withdrawn from the quenched channel member, the female dies expanded, and an unloader mechanism ejects the channel member from the quench machine.

The channel member, after die quenching, can be cooled to room temperature in any desired rate, and the channel member is subsequently subjected to a stress relieving treatment to relieve any residual stresses in the member. The stress relieving treatment generally consists of reheating the member to a temperature up to 1100 F. and usually in the range of 600 to 1000 F. for a period of up to 30 minutes, depending on the properties desired.

FIGS. 12 illustrate the reduction in the variation of dimensional growth brought out by the process of the invention. The curves shown in FIGS. 1-2 are normal frequency distribution curves made in accordance with the formulae shown in Analysis and Correlation of Test Data Irving W. Burr, Metals Progress September 1952. FIG. 1 shows the dimensional variations of carbon steel (A. O. Smith 112A, which has the following specification: 0.22-0.27% carbon, 0.851.20% manganese, 0.04% max. phosphorus 0.05% max. sulfur, balance iron) channels having a thickness of inch and a length of 30 feet when subjected to a single step quench. The channels were initially heated to 1750 F., held as this tem perature for 22 minutes and then die quenched with water to 500 F. in 1.2 seconds. The channels were subsequently stress relieved by heating in a furnace to 900 F. with a total cycle time of 30 minutes.

FIG. 1 shows that 272 pieces were subjected to this treatment and the average dimensional growth (shown by the peak of the curve) was 0.0009 inch per inch. The spread of dimensional change was from 0.0004 inch per inch (indicating a shrinkage) to +0.0022 inch per inch, or a total spread or variation of 0.0026 inch per inch.

FIG. 2 illustrates a similar type curve for channels which were subjected to the two-step cooling procedure of the invention. The channels were of the identical size and composition to those tested in FIG. 1 and were initially heated to 1750 F. and held at this temperature for 3 minutes. The channels were then furnace cooled to 1450" F. in 5 minutes, held at this temperature for 4 minutes, and then die quenched with water to 450 F. in 1.1 seconds. The die-quenching apparatus was similar to that previously described. The channels were subsequently stress relieved by heating in a furnace 900 F. with a total cycle time of 30 minutes.

FIG. 2 shows that 20 pieces were subjected to this two-step cooling treatment and the average dimensional growth was 0.0006 inch per inch considerably less than the average growth which occurred with the single-step quench, as shown in FIG. 1. More important, was the reduction in variation in dimensional change brought about by the two-step quench. In FIG. 2, the spread of dimensional change was from -0.0001 inch per inch to +0.0013 inch per inch, or a total variation of 0.0014 inch per inch, as compared with a variation of 0.0026 inch per inch with the single-step quench shown in FIG. 1.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

We claim:

1. A method of heat treating elongated structural steel members to minimize warpage and dimensional growth, comprising the steps of heating a structural steel member having a carbon content of less than 0.30% to a temperature above the upper critical temperature, holding the member at said temperature for a period of time sufficient to obtain a uniform distribution of temperature, cooling the member to a temperature in the range of 1300 to 1650 F., holding the member at said second named temperature for a period of time sufiicient to obtain a uniform distribution of temperature, and thereafter die quenching the member to a temperature below 800 F.

2. A method of heat treating elongated structural steel members to minimize warpage and dimensional growth, comprising the steps of heating a structural steel member having a carbon content of less than 0.30% to a temperature above the upper critical temperature, holding the member at said temperature for a period of time sufficient to obtain a uniform distribution of temperature, cooling the member to a temperature in the range of l300 to 1650 F., holding the member at said second named tem perature for a period of time sufiicient to obtain a uniform distribution of temperature, positioning the member within a die, and quenching the member while held in the die to a temperature below 800 F.

3. A method of heat treating elongated structural steel members to minimize warpage and dimensional growth, comprising the steps of heating a structural steel member consisting essentially of .15 to 30% carbon, .85 to 1.50% manganese and balance iron to a temperature in the range of 1600 to 1900 F., holding the member at said temperature for a period of time sutficient to obtain a uniform distribution of temperature, cooling the member to a temperature in the range of 1300 to 1650 F., holding the member at said last named temperature for a period suflicient to obtain a uniform distribution of temperature, and thereafter die quenching the member to a temperature below 800 F. at a rate of 400 F. to 1600 F. per second.

4. A method of forming an elongated vehicle frame side rail comprising the steps of forming a flat steel blank having a carbon content less than 0.30% into a channel member having a U-shaped cross section, heating the channel member to a temperature in the range of 1600 to 1900 F., holding the channel member at said temperature for a period of time suflicient to obtain a uniform distribution of temperature, cooling the channel member to a temperature in the range of 1300 to 1500 F., holding the channel member at said last named temperature for a period of time just sufficient to obtain a uniform distribution of temperature, clamping the channel member in a die while at said last named temperature to straighten the channel member, and thereafter water 5 quenching the channel member to a temperature below 800 F.

5. A method of forming an elongated vehicle frame side rail comprising the steps of punching a series of connecting holes in a steel blank having a total alloy content less than 3% by weight and having a carbon content less than 0.30% and a thickness less than /2 inch, forming the blank into a channel member, heating the channel member to a temperature in the range of 1600 to 1900 F., holding the channel member at said temperature for a period of time sufiicient to obtain a uniform distribution of temperature, cooling the channel member to a temperature in the range of 1300 to 1650 F., holding the channel member at said last named temperature for a period of time just sufficient to obtain a uniform distri- References Cited by the Examiner UNITED STATES PATENTS 9/1964 Williams et al. 1483l DAVID L. RECK, Primary Examiner.

) C. N. LOVELL, Assistant Examiner.

Claims (1)

1. 3. A METHOD OF HEAT TREATING ELONGATED STRUCTURAL STEEL MEMBERS TO MINIMIZE WARPAGE AND DIMENSIONAL GROWTH COMPRISING THE STEPS OF HEATING A STRUCTURAL STEEL MEMBER CONSISTING ESSENTIALLY OF .15 TO 30% CARBON, .85 TO 1.50% MANGANESE AND BALANCE IRON TO A TEMPERATURE IN THE RANGE OF 1600* TO 1900* F., HOLDING THE MEMBER AT SAID TEMPERATURE FOR A PERIOD OF TIME SUFFICIENT TO OBTAIN A UNIFORM DISTRIBUTION OF TEMPERATURE, COOLING THE MEMBER TO A TEMPERATURE IN THE RANGE OF 1300* C TO 1650* F., HOLDNG THE MEMBER AT SAID LAST NAMED TEMPERATURE FOR A PERIOD SUFFICIENT TO OBTAIN A UNIFORM DISTRIBUTION OF TEMPERATURE, AND THEREAFTER THE QUENCHING THE MEMBER TO A TEMPERATURE BELOW 800* F. AT A RATE OF 400* F. TO 1600* F. PER SECOND.

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