US3444008A - Controlled atmosphere processing - Google Patents

Description

May 13, 1969 w. R. Ki-:OUGH

CONTROLLED ATMOSPHERE PROCESSING Filed May 9, 1966 w e n ,c .m .m im ,m m. .5 M m I o 5 0 mi. U m .w n v w .mlvm mlv/n 'vm M w. .m 2 Mu w m We .mm u .r Mw im P Ot O H om Us H V I United States Fatent 3,444,008 CONTROLLED ATMOSPHERE PROCESSING William R. Keough, Birmingham, Mich., assignor of fortyve percent to Multifastener Company, Detroit, Mich., a partnership Filed May 9, 1966, Ser. No. 548,554 Int. Cl. C21d 7/14, 1/60, 9/52 U.S. Cl. 14S-12.4 9 Claims ABSTRACT F THE DISCLUSURE A method of heat treating steel objects which includes heating the steel to an austenitic state, quenching the object in liquid to reduce the temperature of the object to a lower temperature at which the steel remains austenitic for a substantial time and then transforms to another crystalline form with the quenching being done suiiiciently fast to avoid transformation of the steel to another crystalline form, working the object at the lower temperature in said liquid, maintaining the object at the lower temperature while the steel thereof transforms to the other crystalline form, and ultimately cooling the object to room temperature.

This invention relates to methods of heat treating steel, and more particularly to a heat treatment involving quenching of austenitic steel and working of the quenched steel while it remains in an austenitic stage so as to greatly increase the tensile strength and ductility of the steel without introducing internal stresses.

There are many applications for steel of high tensile strength and great ductility. Considering only steel wire as an example, such properties are desired for bridge cables, elevator cables, cranes, hoists, and other applications where heavy loads are carried by the wire. Hot working and cold working of steel are well known ways of increasing its tensile strength. In the case of hot working, the grains of the steel are round and this is not the optimum condition for high tensile strength. Steel having elongated grains can be obtained by cold working, and such elongation of the grains increases the tensile strength of the steel. However, cold working introduces internal stresses in the material which detract from its tensile strength and tends to make the steel more brittle. Furthermore, cold working elongates only the grains near the surface of the material.

The method of heat treating steel according to the present invention elongates the grains of the steel throughout the material and results in steel having high tensile strength and also great ductility. In achieving the elongation of the grains of the steel, internal stresses are not introduced into the steel, and thus the method of the invention is superior to cold working.

Briey, the method of the invention includes the steps of:

1) Heating the steel to a temperature at which it becomes austenitic;

(2) Cooling the steel sufficiently fast to avoid transformation ofthe austenite to a lower temperature at which the austenite transforms to bainite;

(3) Working the steel at the lower temperature while the steel remains austenitic;

(4) Maintaining the steel at the lower temperature while it transforms to bainite; and

(5) Ultimately cooling the steel to room tempearture.

The rapid cooling of the steel can be done by quenching it in a liquid maintained at the temperature for transformation to bainite. The latter temperature is above the temperature at which martensite starts to form. The working can be done in the case of wire stock by rolling the wire between dies to reduce its cross-sectional area. The

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transformation of the steel from austenite to bainite after quenching is an isothermal process which in itself hardens and strengthens the steel, and the working of the steel at the transformation temperature cooperates with the quenching and transforming steps to achieve a still higher tensile strength and greater ductility.

It is, therefore, an object of the present invention to provide a method o-f heat treating steel which results in steel having high tensile strength and great ductility.

Another object of the invention is to combine working of steel objects with quenching and isothermal transformation steps so as to produce steel of high tensile strength and great ductility Without introducing internal stresses into the steel.

A further object of the invention is to provide a method of heat treating steel wherein austenitic steel is quenched to a tempearture for isothermal transformation to bainite and is worked while the steel remains in the austenitic stage to obtain a final product characterized by elongated grains free of internal stresses.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

On the drawings:

FIGURE 1 is an isothermal transformation diagram illustrating the heat treatment of the invention;

FIGURE 2 is a ilow diagram of the heat treatment of the invention;

FIG. 3 represents a photomicrograph of hot-worked steel showing round grains; and

FIGURE 4 represents a photomicrograph of steel treated by the method of the invention and having elongated grains.

Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

As shown on the drawings:

The method of heat treating steel according to the invention will be described with reference to the isothermal transformation diagram shown in FIGURE 1. The diagram gives only one example of austenite transformation as it occurs in a particular steel, but it is believed that this example will serve to illustrate the factors which are taken into consideration in planning a heat treatment in accordance with the invention.

In the diagram of FIGURE l, time is plotted in seconds on a log scale along the abscissa axis, and temperature is plotted in degrees F. along the ordinate axis. Two temperatures labeled AS and Af are represented by horizontal lines near the top of the diagram.

The As temperature is that temperature at which a trace amount of austenite forms in the ferrite matrix and does not increase perceptibly in amount when the holding time at As is doubled.

The Af temperature denotes the maximum temperature at which a barely detectable vamount of ferrite can exist in the steel. These temperatures are the limiting or ceiling temperatures for isothermal transformation. Austenite is a solid solution of Fe3C in gamma iron produced by very drastic quenching of carbon steel. It is soft, nonmagnetic and is the densest constituent of any steel.

Near the bottom of the diagram, there is a horizontal dashed line, labeled Ms, which indicates the temperature at which martensite starts to form on quenching from the austenitizing temperature.

There are three curves 10, 11 and 12 in the diagram. Curve 10, which is the first curve from the left, is called the beginning line and represents the time required for a measurable amount of austenite to transform. The second curve 11 represents the beginning of transformation to ferrite-carbide aggregate. The curve 12 farthest toward the right represents the time required at each temperature for the last trace of austenite to transform. Each lield on the diagram above Ms is labeled to indicate the phases observed in specimens austenitized and then quenched and held isothermally within the time-temperature limits of each iield. In the diagram, A is austenite, F is ferrite, C is carbide, and M is martensite. In general, acicular aggregates classied as bainite form from the nose temperature down to Ms. Bainite is a dark etching acicular aggregate obtained by the isothermal transformation of austenite between 400 F. and 1000 F. This temperature range is just above the temperature to which the steel must be quenched to form martensite.

The line drawn through points W, X, Y and Z is the cooling curve (generally designated 14) for a heat treatment in accordance with the invention. The irst step of that method is to heat the steel to a temperature at which it becomes austenitic throughout. For plain carbon steels, this temperature may abe in the range from about 1400 to about 1700 F. The cooling curve 14 of the diagram is for a 1060 steel containing about 0.64 percent carbon and 1.13 percent manganese, and the austenitizing temperature is about 1500 F.

After the steel has been austenitized, it is cooled fast enough to avoid any transformation of the austenite to a lower temperature (represented by point X on the diagram) at which the austenite transforms to bainite. This portion of the cooling curve is represented by the line between points W and X in the diagram of FIGURE 1, and it may be seen that this line lies to the left of the beginning line for transformation, curve 10, such that there is no transformation of the austenite during the rapid cooling step.

The rate of cooling will vary from composition to composition, but the cooling line 14 must be kept to the left of the nose of the beginning line in order to avoid transformation during cooling of the steel.

The rapid cooling may be carried out by quenching the steel in a salt medium which is maintained in the lower temperature X for transformation of the austenite to bainite.

The portion of the cooling curve passing through points X, Y and Z is horizontal, indicating that the steel is held at a constant temperature while it transforms. This transformation temperature is above the Ms line so martensite does not form. In the heat treatment of the invention, the rapid cooling is always terminated at a temperature above that at which martensite starts to form. The steel is not merely held at this transformation temperature, but rather it is worked while the steel remains austenitic and before the transformation to bainite begins. The portion of curve 14 where working takes place is between points X and Y as indicated on a diagram` of FIGURE l, and it is clear that the working is done before transformation begins. The transformation begins at point Y and is complete at point Z. Thus, two steps take place between points X and Z of the diagram. The steel is worked at the transformation temperature (between points X and Y) to elongate the grains of the steel, and thereby increase its tensile strength and ductility, and the steel is maintained (between points Y and Z) at a constant transformation temperature while it transforms to bainite. After the steel has transformed, it may be cooled to room temperature.

The working step is carried out while the steel is in a salt quenching bath. In the case where steel wire is treated by the method of the invention, the wire may be passed through a reduction die in the salt bath which may be .4 a series of rollers mounted with their axes offset at to each other. The rollers achieve an overall reduction in the diameter of the wire. The reduction may be about 30 to 40 percent, or whatever is needed to obtain elongation of the grains of the steel throughout the structure.

FIGURE 4 shows the type of elongated grain structure which is desired, and for purposes of comparison FIGURE 3 illustrates a sample having round grains, such as might be obtained by hot working of steel. It may be seen in FIGURE 4 that the elongation of the grains takes place throughout the material rather than just at the outside of the material, as is the case where steel is cold worked.

The following examples give the times and temperatures for the various portions of the heat treatment for 1050 and 1060 steels.

1050 MATERIAL The heat treatment of the invention may be applied to steels of varying compositions and it is particularly advantageous as applied to plain carbon steels, especially in the form of steel wire. The steel may contain a minor amount of an alloying element such as silicon, chromium or manganese or mixtures thereof. The invention is not necessarily restricted to treatment of Wire. For example, sheet material and rods might Well be treated by the method of the invention.

'I'he invention provides a method of heat treating steel -in which working is accomplished while an austenitized and quenched steel is held at a transformation temperature such that the nal steel is characterized by elongated grains throughout its microstructure. This imparts a high degree of tensile strength and ductility to the material and does not introduce internal stresses.

I claim:

1. A method of heat treating objects made of steel comprising the steps of heating such an object to an elevated temperature at which the steel thereof becomes austenitic, quenching said object in a salt medium to cool said object to a lower temperature sufliciently fast to avoid transformation of the austenite to another crystalline form, said lower temperature being one at which the steel remains austenitic for a substantial time and then transforms to another crystalline form, working said object at said lower temperature while said object is in said salt medium and while said steel remains austenitic in order to increase the tensile strength and ductility of the final steel after heat treatment, maintaining said object at said lower temperature while the steel thereof transforms to said other crystalline form, and ultimately cooling said object to room temperature.

2. A method as defined in claim 1, in which said other crystalline form is bainite.

3. A method as deiined in claim 1, in which said steel object is wire and in which said working step is carried out by rolling said steel wire to achieve an overall reduction in the diameter of said wire.

4. A method as defined in claim 1, in which said Wonking step includes rolling said steel object to decrease the 5 cross-sectional area of said object and increase the length thereof.

5. A method of heat treating objects made of steel comprising the steps of heating such an object to an elevated temperature at which the steel thereof becomes austenitic, then quenching said object in a liquid medium to cool the same sufliciently fast to avoid transformation of the austenite and to a lower temperature at which the austentite transforms to bainite, working said object at said lower temperature and in said liquid medium before the transformation :to bainite begins, holding said object at said lower temperature and in said liquid medium While the steel thereof transforms to bainite, and ultimately cooling said object to room temperature.

6. The method of claim 5 in which said steel is a plain carbon steel.

7. A method of heat treating objects made of steel comprising the steps of heating such an object to an elevated temperature at which the steel thereof becomes austenitic, quenching said object in a liquid which is maintained at a temperature at which the steel remains austenitic for a substantial time and then transforms isothermally to bainite, said quenching being accomplished in a period of time less than the time required for transformation of the austenite to another form, working said object at said lower `temperature in said liquid while said steel remains austenitic and before transformation thereof begins in order to increase the tensile strength and ductility of the steel, maintaining said object at the ternperature of said liquid while the steel thereof transforms to bainite, and ultimately cooling said object to room temperature.

8. The method of claim 7 in which said object is reduced in cross-sectional area by said working step.

9. The method of claim 3 in which said steel object is wire and said working step is carried out by rolling the wire to reduce the cross sectional area thereof.

References Cited UNITED STATES PATENTS 2,435,511 2/1948 Rice 14S-12.4

L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

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