MXPA00000476A - Screws having selected heat treatment and hardening - Google Patents

Abstract

A carbon steel screw has one or more portions which have been selectively hardened by selective heat treatment and quenching. In one embodiment, an upper portion (24) of the screw head (12) is selectively hardened to prevent or reduce damage when torque is applied using a driving tool. The screw tip (16) may be selectively hardened for more effective penetration into a substrate. Preferably, the selectively heated portions are selectively quenched to reduce or avoid distortion.

Description

SCREWS HAVING SELECTED PORTIONS SUBJECTED TO THERMAL TREATMENT AND HARDENING FIELD OF THE INVENTION This invention concerns carbon steel screws and similar fasteners having selectively hardened portions to create the desired properties and behavior, and a method for making the screws selectively hardened. BACKGROUND OF THE INVENTION Stainless steel screws with selectively hardened portions are known from U.S. Patent 3,376,780, issued to Tanczyn. Tanczyn discloses a stainless steel screw having selectively hardened screw pitch ridges and a selectively hardened head portion for insertion of a screwdriver. These portions are harder than the rest of the screw portions. The stainless steel screw has a carbon content not exceeding 0.20% by weight, a chromium content of 10-25% by weight, a nickel content of 5-20% by weight, a copper content of 1-5 % by weight and an aluminum content of 0.25-2.5% by weight. The hardening is carried out by cold rolling the stainless steel at about 371-482 ° C and by aging hardening at about 565-676 ° C. Hardening is greater in the larger portions cold rolled. U.S. Patent 4,295,351, issued to Bjorklund et al., Discloses a stainless steel screw whose crests were selectively hardened. The selective hardening is obtained through the aggressive cold rolling of the targets of the precursor fastener, at temperatures below zero, during the formation of the threads. U.S. Patent 4,289,006, issued to Hallengren, disclosed another selectively hardened stainless steel screw. U.S. Patent 2,229,565, issued to Hallowell Jr., discloses a socket head screw whose portion of the head was selectively hardened. The portion of the screw head is rapidly heated by induction at an elevated temperature. After all the screw is quickly cooled in water, causing hardening in the head portion. The resulting screw may have a Rockwell "C" ("Rc") hardness of approximately 48-50 in the head portion and an Rc of approximately 30-35 in the other portions. U.S. Patent 5,755,542, issued to Janusz et al., Discloses a screw having selectively hardened threads at a lower end of the screw shank, and a selectively hardened tip. U.S. Patent 5,605,423, issued to Janusz, discloses a headless bolt with selectively hardened threads on a lower end of the headless bolt, and a selectively hardened tip. Regular stainless steel screws (with only one slot in the head) and cross screws (with two grooves in the head that intersect) can only be exposed to a torsional moment of a driving tool (for example a screwdriver). When the grooves of the head are exposed to excessive rotational force, they become enlarged and damaged, so that the driving tool can no longer effectively engage the grooves. It has been considered to harden the portion of the head of the screws to strengthen the grooves. However, hardening can cause the head and upper portion of the spindle to become excessively brittle, resulting in 1) the head breaking off the screw shaft when excessive rotation force is applied, 2) fragility by hydrogen absorption if the screws are plated, and 3) head detonation caused by expansion and thermal contraction of the substrates to which the screw is applied, which creates tension that can not be released by lengthening the screw. In addition, selective heating of the head portion to cause hardening may result deformation of the screw when the entire screw (with a variable temperature profile) is exposed to a fast cooling liquid. SUMMARY OF THE INVENTION The present invention is directed to a selectively hardened carbon steel screw having a differential hardness profile within the head portion. A screw that has a head portion is provided, a spike portion below the head and a lower end portion or tip. The head portion has an upper surface, a lower surface, a center an outer flange and at least one slot in the center for coupling a driving tool. The invention also comprises a carbon steel screw having a selectively hardened tip which facilitates the initial penetration of the screw into a substrate. According to the invention, the portion of the head is selectively hardened at the center and at the top so that the center of the portion of the head near the top is more resistant than the bottom of the portion of the head. head and the adjacent shank of the screw. In other words, the edges and walls defining the groove selectively harden at the top to provide strength and hardness and reduce the damage that is caused by a driving tool.

Although the lower portion of the head portion and the adjacent peg remain relatively soft and flexible, the head portion does not separate from the peg when a lot of torque or high tension is applied, such as shear stress. The invention also includes a method for selectively hardening the head in the center and near the top. A source of heat, which may be a flame ejector, is applied directly to the top and center portion of the head portion, causing the portion to reach a temperature above 760 ° C. The maximum temperature reached in the upper and central part of the head is greater than the temperature reached in the lower part of the head or the adjacent portion of the screw shank. Then, the screw can be differentially cooled rapidly to reduce or prevent deformation. The differential fast cooling can be carried out by directing a fast cooling liquid directly to the upper center of the head, thus achieving maximum rapid cooling in the hottest portion. Then the quench liquid can be allowed to flow from the head to the rest of the screw parts, where less rapid cooling is desired. The invention includes a similar technique for selectively treating with heat and cooling Fast to the tip of a screw, causing localized hardening. BRIEF DESCRIPTION OF THE ILLUSTRATIONS Fig. 1 is a sectional view of a screw of the invention with selected hardening in the portion of the head. Fig. 2 is a vertical view of the screw of Fig. 1. Fig. 3 is a vertical view of another embodiment of the screw of the invention. Fig. 4 illustrates a piercing tip screw that can be selectively hardened according to the invention. Fig. 5 illustrates a hexagonal head screw that can be selectively hardened according to the invention. Fig. 6 is a schematic view of a heating apparatus for making selectively hardened screws. Figs. 7 to 11 are sectional views taken along lines 7-7, 8-8, 9-9, 10-10 and 11-11 in FIG. 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS THE PRESENT. With reference to Fig. 1, a carbon steel screw 10 of the invention has a head portion 12, a pin 14 below the head, and a screw tip 16 at one end of the spike as opposed to the head. A plurality of screw passages or threads 18 with peaks 20 and valleys 22 therebetween, surrounding the outer flange of the pin 14 and the tip 16. With reference to Figs. 1 to 3, the portion of the head 12 has an upper surface 24, a lower part 26, a central portion 28 and an outer flange 30. The central portion 28 includes or a single slot 32 for receiving a standard screwdriver or a similar tool , or a pair of transverse slots 32 and 34 for receiving a Phillips ™ screwdriver or a similar tool designed for cross screws. Each groove is defined by a pair of generally vertical opposed walls 36 and 38 which cross the upper surface 24 at the edges 40 and 42, and which also cross the floor of the groove 44 at the joints 41 and 43. The depth of the groove is the distance between the ridges 40 and 42 and the floor of the slot 44. In fact, the floor of the slot 44 can be located - below the screw head 12 and in the adjacent shank 14, indicating a slot depth greater than thickness of the head of the screw as shown in Fig. 1. Alternatively, the floor of the slot 44 may be almost flush with the bottom 26 of the head 12, or it may be above the bottom 26 of the head 16, in which case slot 32 (or slots 32 and 34) is located completely within the head portion 12. The screws 10 of the invention selectively harden to create a differential hardness profile within the head portion 12. A flame ejector or other heat source is applied directly to the upper surface 24 around the central portion 28, so that the heat is higher at the higher points near the center, mainly the edges 40 and 42 of the slot 32 (or the slots 32 and 34). Edges 40 and 42 are heated to a temperature of at least about 760 ° C, preferably about 815-1093 ° C, but better still about 871-982 ° C. The heat causes the affected portions (on and around the edges 40 and 42) to be transformed from a metallurgical ferritic metallurgical structure to an austenitic metallurgical structure. The heat is directed for a sufficiently short period of time so that the bottom 26 of the head 12 and the adjacent portion of the arrow 14 do not undergo this transformation. The screws 10 are then rapidly cooled by directing a quench liquid to the portion of the head 12 that experienced the highest heat. The quench liquid can be water or another liquid or gas, and can be poured, sprayed, sprayed with air, or applied in another form directly to the upper surface 24 in the vicinity of the central portion 28. The applied quench liquid can then flow down to the head, so that there is a less rapid cooling impact on the portions of the screw who experienced less heating. Selective quenching causes the hottest parts (on and around edges 40 and 42) to transform from the austenitic metallurgical structure to a martensitic structure, which hardens but is not tempered. The lower part 26 of the screw head 12, and the adjacent part of the spigot 14, remain substantially in the perliferous ferritic state, which is softer and more flexible. By rapidly and selectively cooling the hottest portion of the head 12 to a greater extent than the colder portions, the deformation of the screw by rapid cooling is minimized. The quench liquid may have a temperature of about 4 to 93 ° C, preferably about 10-68 ° C, but better still about 16-38 ° C. Running water or other process water is adequate. Another means of rapid cooling may include oil or gas. The rapid cooling time does not need to be more than about 30 seconds, and it can be about 3 to 10 seconds. The screw head can continue hot after rapid cooling to facilitate drying. The resulting selectively hardened carbon steel screw has a hardness differential of at least about 10 Rockwell units UC "(" Rc ") within the head portion itself The ridges 40 and 42 near the center of the head 12 they must have an Rc of at least about 45, preferably at least about 50 or better still at least about 55. The lower part 26 of the head 12, and the upper portion of the pin 14 must not have an Rc greater than about 35. The remaining portions of the head 12 may have a value of Rc very close to the Rc of the edges 40 and 42, or closer to the Rc of the lower part 26, depending on their proximity to either of the two. For example, the walls 36 and 38 of the slot 32 (or slots 32 and 34) must have an Rc of at least about 45 near the top and near the center, but may have an Rc of about 35 or less near the s joints 41 and 43 of the floor of the slot 44. The floor of the slot 44 can have an Rc of about 35 or less. The upper surface 24 may have an Rc of at least about 45 near the edges and near the center, and may or may not have a lower Rc near the flange 30.

By selectively hardening the screw head 12 in this manner, additional strength and hardness is provided to the groove 32 which reduces deformation and damage when a screwdriver or similar tool is applied with great torsion. By allowing the lower part 26, the head 12 and the adjacent pin 14 to be softer, the possibility of the head breaking due to a high torsion or a high cutting tension is reduced. The invention is particularly useful for ceiling screws, and other screws having a long spike and / or driving on resistant substrates, since these screws are routinely subjected to high levels of shear stress by extreme temperature variations experienced in ceilings. The screw 10 should be constructed of a very low carbon steel. The suitable carbon content should vary from approximately 0.08-0.50% by weight of the steel, with a preferred scale of approximately 0.18-0.35% by weight of the steel. The carbon must be sufficient to facilitate hardening of the steel by heat treatment, but not so high to facilitate hardening by mechanical means during cold rolling of head, tip or screw threads. In other words, the screw 10 of the invention is selectively hardened by heat and preferably not by cold rolling. In another embodiment illustrated in FIG. 4, the selectively hardened screw of the invention may be a piercing tip screw. One type of piercing tip screw 60 includes a hexagonal head portion 62, a threaded shaft 64 that includes one or more spiral threads 66, and a piercing tip 68 that can be used to thread and pierce at least a portion of the screw 60 on a substrate. The portion of the head 62 can be selectively cured on its outer faces using the techniques described above to obtain improved strength and integrity. In addition, the tip 68 and a portion 67 of the tang 64 that comprise the first threads 66 above the tip 68, can selectively receive heat treatment and harden to facilitate initial penetration of the screw 60 into a substrate, and screwing initial of the threads. A ceiling screw can harden at the tip and just above the tip, in order to overcome the need to drill a hole in the substrate so that the screw can be inserted. With reference to Figure 4, the end portion 67 of the screw 60, defined as the lower portion of the pin 64 adjacent the tip, can be selectively hardened together with the tip 68 by initially applying a flame ejector, or other heat source. , directly to the end portion 67 and the tip 68. These zones 67 and 68 are heated to at least about 760 ° C, preferably at about 815-1093 ° C, and better still at about 871-982 ° C. Selective heating causes the end portion 67 and tip 68 to change from a ferritic metallurgical structure to an austenitic metallurgical structure. Then, the end portion 67 and the tip 68 are selectively quenched rapidly by immersing them or directly directing a quench liquid directly thereto. Selective rapid cooling (which causes uniform cooling around the screw, but rapid differential cooling over its entire length) converts the austenitic metallurgical structure into a martensitic metallurgical structure in the heated portion, which is hard but not tempered. The quench liquid can be water, and can be applied to the aforementioned temperatures and rapid cooling times for the head portion. The end portions 67 and the tips 68 of the screws 60 should harden to an Rc value of at least about 45, preferably at least about 50, and better still at least about 55. The untreated portion of the pin 64 above the end portion 67 may have an Rc value at least about 10 units less than the hardened end portion, and may have an Rc value of about 35 or less, maybe 25 or less.

The hardened portion 67 of the pin can then be annealed to produce a hardness value Rc of about 35-45, which is higher than the start value Rc and still lower than the selectively hardened value. The tempering can be carried out by reheating the selectively hardened portion 67 to about 315 to 593 ° C, preferably to about 399 to 538 ° C. Preferably, the tip 68 is not tempered, but instead remains at its maximum hardness. Spiral threads 66 can also be cemented (ie, hardened externally) to reduce damage when the screw is driven into a substrate. This is particularly useful in the case of long drilling screws used for roofing. In another embodiment, the selectively hardened screw may be a hexagonal head screw having a hexagonal head portion for receiving a driving tool. With reference to Fig. 5, the screw 70 has a head 74, with an upper surface 76. The head 74 also has a hexagonal outer surface 80 composed of six rectangular flat sides 82. In the embodiment shown, a permanent sheave 84 is placed between the head 74 and the elongated threaded pin 86 of the screw. According to the invention, selected portions of the hexagonal head screw 70 can harden using the techniques described above. The upper part of the hexagonal portion 72, which receives the driving tool, can be selectively hardened to provide greater resistance to damage and wear. By hardening the six outer faces 82 of the head portion, the performance of the screw and drive tool can be improved since the interaction is perfected using a hollow drive tool. Again, the piercing tip screw of Fig. 4 and the hex head screw of Fig. 5 can be made of carbon steel having the aforementioned carbon content. Unhardened screw parts may have an Rc value of about 35 or less. The selectively hardened portions may have an Rc value of at least about 45, preferably at least about 50, and better still at least about 55. The portions of the head and end of the screws may be selectively hardened by separated, using different processes, or can receive treatment in a single integral process. Figs. 6 to 11 illustrate schematically an apparatus 100 useful for heat treating selected portions of a large number of screws in a continuous manner. Referring to Fig. 6, the apparatus 100 includes a transport mechanism 102 that assists and moves a screw conveyor 104, which may be a conveyor chain, in the direction of the arrow 106. The screws 101, which may have a variety of lengths, are fastened on the conveyor 104 under their respective head portions 12. The screws 101 are carried on the conveyor 104 through a first heating assembly 108, which includes a plurality of flame burners 110 and thermocouples 112. As illustrated in Fig. 7, the flame burners 110 in an assembly 108. they are used to heat the lower end 46 and the tip 16 of each screw 101, at a temperature, possibly between 871 and 982 ° C. The flame 114 applies only to these selected portions of the screw. An exhaust hood 117 takes away the excessive heat. As the screws 101 pass through the first heating assembly 108, the lower end and the tip of each screw 101 are then rapidly cooled using a cooling assembly 116. As illustrated in Fig. 8, the cooling assembly selectively applies a Water curtain 117, or process oil, or other cooling liquid to the portions of each screw that have been heated. The heated portions are preferably cooled to 68 ° C or less. The first heating set, followed by the By cooling, you can increase the Rockwell Rc hardness of the lower end and tip of each screw, from an initial value of less than 35 (and perhaps less than 25) to a value of approximately 45 or greater (and perhaps 50). or higher). It may be desirable to soften the threads at the lower end to an intermediate hardness, while maintaining the high hardness of the screw tip. This softening, termed "tempering", can be performed by passing the hardened thread portion 46 of each screw through a second heating assembly 120. This second heating assembly 120 can include a plurality of small flame burners 122 which, as illustrated in Fig. 9, they exclusively heat the lower end 46 of each screw, but not the tip thereof, at a temperature of about 399 to 538 ° as determined by the thermocouple sensor 123. This second heating step softens the lower portion 46 of each screw at an intermediate Rockwell Rc value of about 35-45. As the transportation of the screws continues, the portions of the head 12 selectively receive heat treatment using a third heating assembly 124 having one or more flame burners 126 and thermocouple 127. As illustrated in Fig. 10, the burners in the third heating assembly 124 they direct the heat of the flame selectively towards the head portions 12. The head portions are preferably heated to approximately 871-982 ° C. The head portions 12 are then selectively cooled rapidly using a cooling assembly 128 which, as illustrated in FIG. 11, directs water or other quench liquid to the head portions 12. The head portions of that The treatment received may have a Rockwell Rc hardness value of about 45 or more, maybe about 50 or more. The screws 101 can then exit the apparatus 100 for packaging or other use. Although at present preference is given to the embodiments of the invention disclosed herein, various modifications and improvements can be made without departing from the spirit and scope thereof. The scope of the invention is set forth in the appended claims and changes that fwithin the meaning and range of equivalents are considered to be included herein.

Claims (27)

1. CLAIMS 1. A selectively hardened carbon steel screw comprising a screw head, a shank having a first end adjacent to the head and a second end, and a screw tip adjacent to the second end of the shank; the screw head including an upper surface, a lower and a central portion, an outer flange and a cavity in the central portion for receiving a driving tool; the screw head having a martensitic metallurgical structure in the upper part of the central portion and a pearlitic ferritic metallurgical structure in the lower part.
2. 2. The screw of Claim 1, wherein the spike has a pearlitic ferritic metallurgical structure.
3. The screw of Claim 1, wherein the spike has a pearlitic ferritic metallurgical structure at the first end and a martensitic metallurgical structure at the second end, and the screw tip has a martensitic metallurgical structure.
4. The screw of Claim 1, which comprises approximately 0.15-0.50% by weight of carbon in the steel.
5. 5. The screw of Claim 1, which comprises approximately 0.18-0.35% by weight of carbon in the steel.
6. 6. A selectively hardened carbon steel screw comprising a screw head, a shank having a first end adjacent to the head and a second end, and a screw tip adjacent the second end of the shank; the head of the screw includes a selectively hardened upper portion and a lower portion that was not hardened; the spike includes an upper portion that was not hardened; The screw of Claim 6, wherein the tip of the screw was also selectively hardened. The screw of Claim 7, wherein the shank further comprises a selectively hardened lower portion. The screw of Claim 7, wherein the selectively hardened screw tip has a Rockwell hardness "C" of at least about 45. The screw of Claim 9, wherein the Rockwell "C" hardness is at least about 50. The screw of Claim 8, wherein the selectively hardened portion of the spike has a Rockwell hardness "C" of approximately 35-45. 12. A selectively hardened carbon steel screw comprising a screw head, a screw shank having a first end adjacent to the head and a second end, and a screw tip adjacent the second end of the shank; wherein a portion or portions of the screw are selectively hardened by heat treatment at an elevated temperature, followed by rapid cooling with liquid, to produce a Rockwell "C" hardness of at least about 10 units greater than the remainder of the portion or portions of the screw that were not selectively hardened; The screw of Claim 12, wherein an upper portion of the head is selectively hardened. The screw of Claim 13, wherein a lower portion of the head is not hardened. 15. The screw of Claim 12, wherein the tip of the screw is selectively hardened. 16. The screw of Claim 15, wherein the second end of the first pin is selectively hardened and then hardened. 17. The screw of Claim 12, wherein the elevated temperature is at least about 760 ° C. 18. The screw of Claim 12, wherein the elevated temperature is approximately 815 to 1093 ° C. The screw of Claim 12, wherein the elevated temperature is approximately 871 to 982 ° C. 20. The screw of Claim 12, wherein the selectively hardened portion or portions are selectively quenched rapidly by a liquid. 21. The screw of Claim 12, wherein the head comprises a single slot having a hardened upper portion. 22. The screw of Claim 12, wherein the head comprises two intersecting grooves, having hardened upper portions. 23. The screw of Claim 12, wherein the head comprises a hexagonal head. 24. The screw of Claim 23, wherein the hexagonal head comprises a hardened grooved area. 25. The screw of Claim 23, wherein the hexagonal head comprises hardened outer surfaces. 26. The screw of Claim 12, wherein the screw tip comprises a hardened piercing tip. 27. The screw of Claim 12, wherein the spike comprises cemented threads