US3907611A

US3907611A - Method for making ferrous metal having highly improved resistances to corrosion at elevated temperatures and to oxidization

Info

Publication number: US3907611A
Application number: US305558A
Authority: US
Inventors: Takao Sasame; Toshio Yagi
Original assignee: Toyo Kogyo Co Ltd
Current assignee: Mazda Motor Corp
Priority date: 1972-11-10
Filing date: 1972-11-10
Publication date: 1975-09-23
Anticipated expiration: 1992-09-23

Abstract

A method of making ferrous metal having highly improved resistances to corrosion and to oxidization, which comprises a step of hot-dipping a metallic workpiece in a molten aluminum or aluminum alloy bath, a step of primary heat-treatment of the metallic workpiece and a step of secondary heat-treatment of the metallic workpiece. During the primary heat-treatment, an intermetallic compound layer is formed over the surface of the metallic workpiece, which is developed into an alloy layer during the secondary heat-treatment to make the alloy layer firmly secured to the base of the workpiece. The secondary heattreatment may be omitted depending on the application of the ferrous metal processed by this method.

Description

United States Patent [191 Sasame et al.

[451 Sept. 23, 1975 [54] METHOD FOR MAKING FERROUS METAL HAVING HIGHLY IMPROVED RESISTANCES TO CORROSION AT ELEVATED TEMPERATURES AND TO OXIDIZATION [75] Inventors: Takao Sasame; Toshio Yagi, both of Hiroshima, Japan [73] Assignee: Toyo Kogyo Co., Ltd., Japan [22] Filed: Nov. 10, 1972 [21] Appl. No.: 305,558

[52] US. Cl. 148/6.3; 29/195; 29/1961; 29/1962; 148/31.5 [51] Int. Cl. C23C 11/00; B23P 3/00 [58] Field of Search 148/6.3, 6.35, 31.5; 117/114 C;29/l96.1,196.2, 195

[56] References Cited UNITED STATES PATENTS 2,444,422 7/1948 Bradfordmln l48/6.35 X

3,418,174 12/1968 Carter et al l48/6.35 3,660,173 5/1972 Matsumo et al. 148/6.3 X 3,730,758 5/1973 Laidman 117/1 14 C X 3,779,056 12/1973 Radjen et al 117/114 C X Fe-Afilntermetolhc Compound Loyerfl -Bose 3,787,228 l/1974 Rausch et al. 117/114 C FOREIGN PATENTS OR APPLICATIONS 754,592 8/1956 United Kingdom 1,018,628 l/l966 United Kingdom 1,079,687 8/1967 United Kingdom Primary ExaminerThomas J. Herbert, Jr. Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [57] ABSTRACT A method of making ferrous metal having highly improved resistances to corrosion and to oxidization, which comprises a step of hot-dipping a metallic workpiece in a molten aluminum or aluminum alloy bath, a step of primary heat-treatment of the metallic workpiece and a step of secondary heat-treatment of the metallic workpiece. During the primary heattreatment, an intermetallic compound layer is formed over the surface of the metallic workpiece, which is developed into an alloy layer during the secondary heat-treatment to make the alloy layer firmly secured to the base of the workpiece. The secondary heattre'atment may be omitted depending on the application of the ferrous metal processed by this method.

6 Claims, 4 Drawing Figures F/GZ US Patent Sept. 23,1975 Sheet 10f2 3,907,611

Fe-Aelmermetollic Compound Layer-EL Bose *Bcse PAlloy Layer US Patent Sept. 23,1975 Sheet 2 of2 3,907,611

FIG. 3

[A8 Fe-Mlntermetollic F Compound Loyer -\e Base k A -Cr Compound Contained w "at -A 203 (Fe, Cr)

CORROSION AT ELEVATED TEMPERATURES AND 'ITO OXIDIZATION The present invention relates to a method for makin'g ferrous metal'having highly improved'resistances' to corrosion at elevatedtemperaturesand to oxidization and, more particularly, to the method wherein'a ferrous metal workpiece, after having been hot-dipped in a molten metal bath containing therein aluminum or its alloy with chromium, isheat-treated to form an alloy layer 7 on the base of the ferrous metal workpiece thereby to improve theresistances to corrosion'at the elevated temperatures and to oxidization.-

In thefield of autornobile'industry, 'an' after-burner or thermal reactor has been largely employed in an automotive vehicle and ithas' been well recognized as one of effective means "for reducing orsubstantially eliminating noxious unburned compounds present in exhaustgas emerging from the exhaust system of the automotive'vehicle; As has been well understood, the exhaust gas emerging from theautomotive exhaust system contains,'in addition to the unburned compounds such as CO and HC, gaseous halogens and halides and- /or lead compounds suchasCl Br PbCl PbBr C l l Cl and C H Br which are corrosive mediums. In

the presence of such corrosive mediums, various metallic parts disposed in the automotive exhaust system tend to be easily corroded and, in the'case where such an after-burnerin provided in theautomotive exhaust system, in which the interior temperature tends to elevate up to 900C. and, in an extreme ease, up to l,200C., during re-combustion of the unburned compounds-in saidafter-burner, corrosion of the afterburner is accelerated by the elevated temperature, thus reducing the working efficiency of the after-burner while shortening the service time of the after-burner. In view of this, especially the after-burner is required made of speciallydesigned, expensive material having a sufficient resistances to corrosiona't the elevated temperatures as well as to oxidization.

Recently, such a metallic material as having a relatively high corrosion resistance at elevated temperatures as wellas a sufficient resistance to oxidization has been developed in the field of space craft industry. which may be more or less satisfactorily employed in the manufacture of the after-burner or thermal reactor. However, this metallicmaterial developed for the manufacture of a space craft is too expensive to make it available for the manufacture of metallic articles for public useincluding an'automotive vehicle having such an after-burner. l

- Accordingly; an essential object of the present invention is to provide an improved method for making ferrous metal having a relatively high corrosion resistance and resistance to oxidization which can be relatively easily manufactured at low costs.

Another important object'of the present invention is to provide the improved method of the above mentioned character wherein an article made of ferrous metal is first hot-dipped'in a molten metal bath containing therein aluminum or its alloy to form an aluminum foil over the surface-of said article and, then, subjected to a heat-treatment to form an alloylayer on the base of the ferrous metal of said article-thereby to improve the corrosion and oxidization resistances of'said arti- LII 2 cles without requiring a substantially complicated procedure. I

It is a related object of the present invention to providea ferrous metal workpiece manufactured by the above mentioned method.

It is to be noted that the present invention has been primarily developed in view ofproviding a relatively high corrosion and oxidization resistant and inexpensive metallic material for the manufacture of the afterburner. However, themethod of the present invention has many applications and, for example, it is applicable not only to the manufacture of automotive vehicle parts such as after-burner and exhaust muffler, but also to the manufacture of various metallic articles such as plates, pipings, containers, vessels and others, all of which are accessible to corrosive mediums no matter how said corrosive mediums are in gaseous, solid or liquid state and which are made of ferrous metal.

Furthermore. in view of the fact that the ferrous metal processed by the method of the present invention has a relatively high corrosion resistance as well as a relativelyhigh resistance to oxidization, the method of the present invention can be applicable to the manufacture of various articles which require either or both of these characteristics. Moreover, the corrosion resistance of the ferrous metal process by the method of the present invention permits such ferrous metal to be used in the manufacture of various jigs for use in casting of light alloys such as those of aluminum and zinc, which may otherwise be easily oxidized in contact with the molten casting metal.

It is also to be noted that the present invention may be applicable to any type of metallic articles including unprocessed workpieces produced by such methods as rolling, extrusion, drawing, casting and others and intermediate and final products processed by such methods as pressing, welding, grinding and others. In addition thereto, the unprocessed, intermediate or final products, after having been processed by the method of the present invention, may be clectro-plated in any suitable manner.

These and other objects and features of the present invention will become apparent from the following description taken in conjunction with a preferred embodiment thereof with reference to the accompanying drawings, in which;

FIG. 1 is a schematic diagram showing the longitudi nal section of a ferrous workpiece obtained by dipping in a molten metal bath containing substantially pure aluminum and then heat-treating according to the present invention and a graph obtained by the X-ray 'microanalizer of distribution of various elements contained in the ferrous workpiece thus processed,

FIG. 2 is a similar diagram to FIG. 1, but the ferrous workpiece being further heat-treated,

FIG. 3 is a schematic diagram showing the longitudinal section of another ferrous workpiece obtained by dipping in a molten metal bath containing an aluminum alloy including 4 wt% of chromium and then heattreating according to the present invention and a graph obtained by the X-ray microanalizer of distribution of various elements contained in the ferrous workpiece thus processed, and

FIG. 4 is a similar diagram of FIG. 3, but the ferrous workpiece being further heat-treated.

Before the description of the present invention proceeds,it is to be noted that, although various elements contained in an article made of ferrous metal (hereinafter referred to as ferrous workpiece), for example, Cr and Ni in the case where such ferrous workpiece is made in austenitic stainless steel, are in practice distributed in a layer formed on the surface of said workpiece during execution of the methodof the present invention, more or less affecting the characteristics of said layer, the distribution of such elements is not shown in the accompanying drawings because the present invention is to be understood in terms of a relationship between a ferrous substance contained in the workpiece and an aluminum substance and/or a chromium substance contained in the layer.

It is further to be noted that the method of the present invention can be applicable to any workpiece made of any one of ferrous steel, including various carbon steels and such special steels as ferritic stainless steel, martensitic stainless steel and austenitic stainless steel, and cast ironsincluding ordinary cast irons and such special cast irons as ductile cast iron and alloy cast iron.

In other words, it is to be understood that any workpiece made of material having Fe as the principle component can be subjected to the method of the present invention no matter what it contains, in addition to Fe, one or a mixture of numerous inorganic additives.

As the first step of execution of the method of the present invention, the ferrous workpiece is hot-dipped for 30 to 300 seconds in a molten metal bath containing therein aluminum or its alloy and heated to a temperature of from 700 to 950C, thereby to form a metallic foil on the surface thereof. The reason for the employment of aluminum or its alloy for the moltenmetal bath is because of its excellent corrosion and oxidization resistances. f

In this case, if the bath temperature is lower than the lowermost limit of 700C., a satisfactory result of dipping process cannot be obtained and, if it is, higher than the uppermost limit of 950C, dimensional variation of the ferrous workpiece will be observable due to softening of the ferrous workpiece under the elevated temperature and, in an extreme case, it will be partially or wholly melted into the molten metal bath.

Preferably, the use of aluminum alloy is recommended for the molten metal bath, in which case the aluminum alloy must contain chromium in an amount within the range of from 1 to percent by weight based on the total weight of the aluminum alloy. This is because the presence of chromium in the molten metal bath results in an improvement in the smoothness of the surface of the resultant foil adhering to the ferrous workpiece and also in the corrosion resistance with respect to any of lead compounds. However, if the aluminum alloy including chromium in an amount smaller than the lowermost limit of l wt7r is employed, such improvements as hereinabove described cannot be obtained whereas, if the alloy including chromium in an amount greater than the uppermost limit of 10 wt% is employed, the surface of a compound layer as will be mentioned later will be roughened at the time of completion of the subsequent heat-treatment. It is to be noted that, in the case where the molten metal bath is prepared by the use of the aluminum alloy including chromium in the specified amount, the lowermost limit of the bath temperature be preferably fixed at 750C.

With the above in mind, if the dipping hour is shorter than the lowermost limit of 30 seconds, the subsequent heat-treatment subject to the ferrous workpiece having the metallic foil thereon will not result, in satisfactory formation of the compound layer that is, an Al-Fe intermetallic .compoundlayer containing a Fe-Al compound, such asFe Al and Fe Al, as the principle component in a desired depth, whereas, if it is longer than the uppermost limit of 300 seconds, a ferrous substance contained in the ferrous workpiece will be melted into the molten bath, thus reducing the weight of the workpiece to be processed. The dipping hour may vary, within the range of 30 to 300 seconds, depending uponthe thickness and shape of the ferrous workpiece to be processed by the method of the present invention.

Subsequent to the hot-dipping process, the ferrous workpiece having a metallic foil thereon is thensubjected to a primary heat-treatment thereby to cause the aluminum substance in the metallic foil to undergo an intermetallic combination with the ferrous substance in the ferrous workpiece. More specifically, if the substantially pure aluminum is employed for the molten metal bath, only the Fe-Al intermetallic compound layer of 50 to 200 micron in depth which contains the Fe-Al compound as the principle component can be obtained as shown in FIG. 1. On the other hand, if the aluminum alloy is employed for the molten metal bath, as shown in FIG. 3, not onlythe Fe-Al intermetallic compound layer of 50 to 200 micron in depth can be obtained, but also the same intermetallic compound layer contains, in addition to the Fe-Al compound as the principle component, an Al-Cr compound, such as Al Cr and Al -,Cr, distributed substantially in the vicinity of the surface of said intermetallic compound layer with the aluminum substance being stablized therein.

To achieve theoptimum result, the primary heattreatment must be carried out under a temperature of from 700 to 930C. for more than 30 minutes. If the heating temperature is lower than the lowermost limit of 700C, the intermetallic compound layer will not be satisfactorily obtained and, if it is higher than the uppermost limit of,930C., the aluminum substance contained in the foil will not satisfactorily undergo the intermetallic combination with the ferrous substance in the ferrous workpiece during this primary heat treatment and, hence, a satisfactory development of the intermetallic compound layer will not be observable, because of being oxidized into A1 0 without providing the desired corrosion resistance. The heating temperature-may vary, within the range of from 700 to 930C, depending upon the type or kind of the ferrous workpiece to be processed by the method of the present invention. I

In addition, the heating time must be not less than 30 minutes, or otherwise development of the intermetallic compound layer will not be satisfactorily observed. However, the heating may preferably continue for not more than 3 hours in view of economical industrial practice.

This primary heat-treatment may be carried out in any ambient atmosphere except for extremely oxidizing atmosphere.

The art of hot-dipping a ferrous workpiece in the molten metal bath containing aluminum or its alloy for the purpose of improving the resistance to oxidization is heretofore known by those skilled in the art. However, the faet has not yet been known that the resistance to corrosion at the elevated temperature of the ferrous workpiece after having been dipped in the molten metal bath can be improved by heat-treatment under the temperature of from 700 to 930C. for more than 30 minutes. In other words. because a ferrous articlemanufactured merely by dipping it in the molten metal bath is such that the aluminum substance contained in the resultant foil is not distributed so as to combine with the ferrous substancesthe article lacks the sufficient corrosion resistance. Fromthe foregoing. it is clear that the most important feature of the present invention so far described resides in that the ferrous workpiece is. after having been hot-dipped in the molten metal bath. subjected to the heat-treatment under the particular temperature for the particular period of time.

It is also clear that the aluminum substance transplanted to the surface of the ferrous workpiece during the hot-dipping process is uniformly distributed into the workpiece during the primary heat-treatment to provide the intermetallic compound layer which has relatively high corrosion and oxidization resistances. However, this intermetallic compound layer has a somewhat insufficient interlocking with respect to the base of the ferrous workpiece.

Accordingly. in the case where the ferrous workpiece having the intermetallic compound layer is to be used in contact with flowing particles of gaseous corrosive medium and/or to be used in the manufacture of an ar ticle accessible to vibrations. it is recommended to carry out a secondary heat-treatment to permit the intermetallic compound layer to be firmly interlocked with the base of the workpiece. i.e.. to develop into an alloy layer. without substantially reducing the improved corrosion and oxidization resistances.

This secondary heat'treatment is carried out at a temperature lower than the melting point of the ferrous workpiece and within the range of 950 to l.350C. for not more than hours. The primary purpose of this secondary heat-treatment is to diffuse aluminum. which is one of the substances contained in the intermetallic compound layer. into the ferrous workpiece thereby to form the alloy layer having Fe and Al as its principle composition which is firmly interlocked with the base of the ferrous workpiece. During this process. there can be found a tendency that. as diffusion of aluminum proceeds. the content of aluminum in the surface portion of the resultant alloy layer reduces with the result of reduction of the corrosion resistance. In order to avoid this tendency. the secondary heattreatment must be carried out in the oxidizing atmosphere.

If the secondary heat-treatment is carried out in the oxidizing atmosphere as hereinbefore described. the aluminum present in the surface portion of the resultant alloy layer is combined with oxide present in the acidic atmosphere thereby to form a compound such as aAl- ,O without reducing the density of aluminum present in said surface portion. In addition thereto. in view of the fact that the primary heat-treatment is cf fective to form the Fe-Al intermctallic compound layer. Fe acts as a binder. even if the aluminum present in said surface portion is oxidized as hereinbefore described. so that a layer of CK-AI2O3 compound can be firmly secured in the resultant alloy layer. Accordingly. the improved corrosion and oxidization resistances obtainable by the method of the present invention does not substantially reduce. It is to be noted that. in the case where the molten metal bath contain the aluminum alloy including chromium. chromium also acts as a binder in the surface portion of the resultant alloy layer in cooperation with Fe.

To achieve the optimum result ofthe secondary heattreatment. the heating temperature must be within the range of from 950 to l.350C. as stated above. If this temperature is lower than the lowermost limit of 950C. satisfactory diffusion of the aluminum will not be observable and. hence. the alloy layer containing Fe and Al as its principle components will not be satisfactorily formedwith no substantial improvement of the interlocking with the base ofthe ferrous workpiece. On the other hand. ifthe temperature is higher than the up pcrmost limit of 1.350C.. not only the aluminum present in the surface portion of the resultant alloy layer is oxidized into a-AI- O but also Fe. which acts as a binder. is oxidized and, therefore. the corrosion resistance and the resistance to oxidization will be reduced. In any event. this heating temperature may vary. within the range of from 950 to l.350C.. depending upon the melting point of the ferrous workpiece to be processed. In other words. the heating temperature must be lower than the melting point of the workpiece to be processed. For example. in the case where the ferrous workpiece is made of cast iron. it must be lower than l.l00C.

In addition thereto. the heating hour must be not more than 10 hours. or otherwise Al and Fe are similarly oxidized. respectively. with substantial reduction .of the corrosion resistance as well as the resistance to oxidization. The minimum permissible heating hour during the secondary heat-treatment is not limited because of the fact that the ferrous workpiece that has been subjected up to the step of primary heattreatment can be placed in practical use without subjecting to the secondary heat-treatment. However. in the case where the secondary heat-treatment in the manner as hereinbcfore described is to be carried out. at least 30 minutes or more is required. It seems. from the foregoing. that the heating hour is limited within the range of 30 minutes to 10 hours and this is' true in the case where the secondary heat-treatment is carried out in the usual atmosphere. However. in the case where the secondary heat-treatment is carried out in the oxidizing atmosphere. the heating hour may be shorter than the minimum permissible value of 30 minutcs.

The alloy layer thus obtained on the ferrous workpiece upon completion of the secondary heattreatment represents such structures as shown in FIGS. 2 and 4 and each of which extends toward the base in a depth of from 200 to 500 micron. the structure of FIG. 2 being the case where the ferrous workpiece has been dipped in the molten metal bath of substantially pure aluminum while the structure of FIG. 4 being the case where the ferrous workpiece has been dipped in the molten metal bath of the aluminum alloy with chromium.

From FIG. 2, it is clear that the alloy layer contains (it-A1 0 rigidly secured by the binding action of Fe in the vicinity of the surface portion thereof with the aluminum content reducing toward the base. The structure of FIG. 4 is similar to that of FIG. 2, but the ot-Al O compound is rigidly secured by the binding ac tion of Fe and Cr while the contents of Al and Cr respcctively reduce toward the base. More particularly. in the structure of FIG. 4, the alloy layer comprises a surface portion containing Fe. Al and Cr and a portion adjacent to the base containing Fe and Al.

In either of the structures of FIGS. 2 and 4. the depth of the surface portion of the alloy layer including the (X-Al is approximately to 80 micron.

From the foregoing. it has now become clear that. by the secondary heat-treatment, (It-Alg0 is formed in the surface portion of the alloy layer as secured by the binding action of Fe and. consequently. the improved corrosion and oxidization resistances can be obtained. In the case where the alloy layer contain chromium because of the use of the aluminum alloy for the molten metal bath, not only this chromium cooperate with Fe as a binder, but also the presence of chromium improves the corrosion resistance to the lead compounds and, accordingly, this ferrous material having the alloy layer containing chromium can be advantageously utilized in the manufacture of the after-burner and exhaust muffler. Furthermore, the secondary heattreatment permits aluminum to diffuse into the ferrous taining, in addition to Fe and Al as its principle components, such elements as used as the additives. Accordingly, no substantial reduction of the corrosion and oxidization resistances occur even in the presence of the additives in the ferrous workpiece. Byway of example, in the case where the ferrous workpiece is made as austentic stainless steel, chromium and nickel contained in According to the present invention, it has been found that, if the secondary heat-treatment is carried out subsequent to the primary heat-treatment which has been carried out under the temperature within the range of 700 to 930C. for more than minutes thereby to permit the aluminum to be stabilized, formation of a-AI Q, takes place in an individually scattered manner and, consequently, no substantial peeling of the alloy layer as hereinbefore described takes place.

Hereinafter, the present invention will be illustrated by way of example.

said austentic stainless steel are diffused into the alloy layer during the secondary heat-treatment, thereby forming the alloy layer containingsuch elements in adstantially intermediate portion of the resultant alloy layer during the secondary heat-treatment. If this for- Five groups of four sample pieces A, B, C and D were tested. These four sample pieces, A, B, C and D are made of ductile cast iron (globular graphite cast iron), low carbon steel containing 0.1% of carbon, I7Cr steel (one of ferritie stainless steels) and l8Cr-8Ni steel (one of austenitic stainless steels). It is to be noted that l7Cr steel is specified by "SUS 24" according to the Japanese Industrial Standard G 4304-9 which is an equivalent of l7Cr steel as specified by Type No. 430" according to the American Iron and Steel Institute or X8 Crl7f as specified by No. 4016 according to Deutsche Industrie Normung, and l8Cr-8Ni steel is specified by SUS 27" according to the Japanese Industrial Standard G 4303-9 which is an equivalent of 18-88" as specified by Type No. 304" according to the American Iron and Steel Institute or X5 CrNi l8 9 as specified by No. 430] according to Deutsche Industrie Normung.

The test 'resuts are shown in the following Table wherein the five groups are clasified by I. II, III, IV and V. Of them, the four. sample pieces A, B. C and D under Group I were submitted to the test without being processed by the method of present invention while those under Groups II and V were processed by the method of-the present invention as exemplified in the Table mation of a-Al o takes place in a relatively great prior to the test.

TABLE Primary Heat- Secondary Heat- Weight Reduction Rate Sample Dipping Process Treatment under Treatment under (71) Piece Bath Temp. Type of Atmosphere Atmosphere at at at Groups Codes (C) Molten Metal Temp.(C.) Hour Temp.(C.) Hour 900C. l.000C. l.l00C.

A 32 60 B 25 I Not Processed Not Treated Not Treated C I5 24 78 D l3 2I 58 A 800 I Not Treated 3 8 Pure B 750 aluminum 800 1 Not Treated 2 6 II 1 minute dipped C 780 1.5 Not Treated l 4 [3 D 770 l 5 Not Treated 0 3 6 A 800 l l .100 1.5 5 l3 Pure B 750 aluminum 800 I 1.100 2 3 ll Ill 1 minute dipped C 780 L5 l,l()() 2 2 6 l7 D 770 1.5 H50 l l 5 7 Table -.Continued l'rimary Heat- Secondary Heat- Weight Reduction Rate Sample Dipping Process Treatment under Treatment under- (71) Piece Bath Temp. Type of Atmosphere Atmosphere at at at .Groups Codes ("C) Molten'Metal Temp.(C.) Hour Tcmp.(C.) Hour 900C. 1.000C. 1.100C.

A 800 1 'Not Treated 3 aluminum alloy with 800 I Not Treated 0 2 IV- 830 4 chromium C 1 minute dipped 780 1.5 Not Treated 0 0 l D 770 1.5 Not Treated 0 0 2 aluminum B alloy with 800 1 1.100 2 0 4 w V e 830 47: chromium C 1 minute clipped 780 1.5 1.100 2 0 0 2 D 770 1.5 1.150 1 O 0 2 The test were carried out in the following manner: Each sample pieces under Groups 1 and V was prepared in the form of a plate having 3 mm in thickness.

30 mm in width and mm in depth and was horizontally placed on a test bench. A powder of lead halide was spread 2 mm in depth over the upper surface of each of the sample pieces under Groups 1 to V and heated at a predetermined temperature of 900C. 1.000C..and 1.100C. for 2'hours. Each of the test results is tabulated in the Table in termsof percentage of reduction of the weight with respect to the original weight measured prior to'the test.

As to the bondability'of the alloy layer in the'sample pieces A, B, C and D under Groups 11 to V. these samunder-Groups 11 and 1V. This factclearly indicates that the bondability of the alloy layer with respect to the ferrous workpiece can be improved by performing the secondary heat'treatment. irrespective of the presence of chromium in the molten metal bath during the hotdipping process. I v

From the foregoing description, it has now become ,clear that the presence of chromium in the aluminum bath. i.e.. the employment of either pure aluminum or the aluminum alloy for the molten metal bath, affects 1 the improvement in" the corrosion resistance and the secondary heat-treatment in the method of the present invention improves the bondability of the alloy layer to "the ferrous workpiece compared with that without the secondary heat-treatment. 1

In any case, the test results exhibit that the method workpiece as compared with those not processed by the methodof the present invention. 1 j J I s'pecificallyyferrous metallic meterial that has been dipped in the molte'nmetal bath of the aluminum alloy. then subjected to the primary heat-treatment and finally subjected to the secondary heat-treatment is recommendable for the manufacture of the after-burner which has the severe requirements as hercinbefore described.

resistance to oxidization whereas the presence of the 'ofthe present invention is-ve'ry effective to improve the corrosion and oxidization resistances of the-ferrous.

What is claimed is:

1. A method for making ferrous metal having highly improved resistance to corrosion and oxidization. which comprises dipping a ferrous workpiece in a molten metal bath containing an aluminum alloy containing chromium in an amount within the range of l to 10 percent by weight based on the total weight of said aluminum alloy. and having a temperature of from 750 to 930C for from to 300 seconds. heating the resultant ferrous workpiece for at least 30 minutes at an elevated temperature of from 700 to 930C. and heating the thus treated ferrous workpiece at a predetermined termperature within the range of 950 to 1.350C which is lower than the melting point of said ferrous workpiece. under an oxidizing atmosphere for not more than 10. hours. thereby forming an alloy layer uniformly over the surface of said ferrous workpiece. said alloy later containing iron and aluminum as its princi ple components and including a substantial layer of a oz-Al O secured therein by the binding action of said iron. at the surface portion of said alloy laycr. said surface portion containing chromium which acts as a binder for securing said substantial layer of cit-A1 0 in cooperation with said iron.

2. A method as claimed in claim 1, wherein said alloy layer is 200 to 500 microns in depth with said substantial layer of oz-A1 0 extending 10 to 80 mircrons in depth.

3. A method as claimed in claim 1. wherein the minimum heating time during the second heating step is more than 30 minutes.

4. A method for making ferrous metal having highly improved resistance to corrosion and oxidization.

' which comprises coating a ferrous workpiece with an aluminum alloy containing chromium in an amount within the range of l to 10 percent by weight based on the total weight of said aluminum alloy. heating the coated ferrous workpiece for at least 30 minutes at an elevated temperature of from 700 to 930C. and heating the thus treated ferrous workpiece at a predetermined temperature within the range of 950 to l.35(lC which is lower than the melting point of said ferrous workpiece. under an oxidizing atmosphere for not more than 10 hours. thereby forming an alloy layer uniformly over the surface of said ferrous workpiece. said alloy layer containing iron and aluminum as its principle components and including a substantial layer of a a-Al- Q, secured therein by the binding action of said iron. at the surface portion of said alloy layer. said surface portion containing chromium which acts as a binder for securing said substantial layer (via-A1 0 in cooperation with said iron.

5. A ferrous workpiece prepared according to the method of claim 1. having a surface portion formed with an alloy layer having relatively high resistances to corrosion and to oxidization said alloy layer being 200 to 500 microns in depth and containing iron and aluminum as its principle components and including therein a substantial layer of a-Al O of IO to 80'microns in depth secured in a surface portion of said alloy layer, and wherein at least said surface portion of said alloy layer contains chromium which acts as a binder for se-

Claims

1. A METHOD FOR MAKING FERROUS METAL HAVING HIGHLY IMPROVED RESISTANCE TO CORROSION AND OXIDAZATION, WHICH COMPRISES DIPPING A FERROUS WORKPIECE IN A MOLTEN METAL BATH CONTAINING AN ALUMINUM ALLOY CONTAININ CHROMIUN IN AN AMOUNT WITHIN THE RANGE OF 1 TO 10 PERCENT BY WEIGHT BASED ON THE TOTAL WEIGHT OF SAID ALUMINUM ALLOY, HAVING A TEMPERATURE OF FROM 750* TO 930*C FOR FROM 30 TO 300 SECONDS, HEATING THE RESULTANT FERROUS WORKPIECE FOR AT LEAST 30 MINUTES AT AN ELEVATED TEMPERATURE OF FROM 700* TO 930*C, AND HEATING THE THUS TREATED FERROUS WORKPIECE AT A PREDETERMINED TEMPERATURE WITHIN THE RANGE OF 950* TO 1,350*C WHICH IS LOWER THAN THE MELTING POINT OF SAID FERROUS WORKPIECE, UNDER AN OXIDIZING ATMOSPHERE FOR NOT MORE THAN 10 HOURS, THEREBY FORMING AN ALLOY LAYER UNIFORMLY OVER THE SURFACE OF SAID FERROUS WORKPIECE, SAID ALLOY LATER CONTAINING IRON AND ALUMINUM AS ITS PRINCIPLE COMPONENTS AND INCLUDING A SUBSTANTIAL LAYER OF A-A2O3 SECURED THEREIN BY THE BINDING ACTION OF SAID IRON, AT THE SURFACE PORTION OF SAID ALLOY LAYER, SAID SURFACE PORTION CONTAINING CHROMIUM WHICH ACTS AS A BINDER FOR SECURING SAID SUBSTANTIAL LAYER OF A-AI2O3 IN COOPERATION WITH SAID IRON.

2. A method as claimed in claim 1, wherein said alloy layer is 200 to 500 microns in depth with said substantial layer of Alpha -Al2O3 extending 10 to 80 mircrons in depth.

3. A method as claimed in claim 1, wherein the minimum heating time during the second heating step is more than 30 minutes.

4. A method for making ferrous metal having highly improved resistance to corrosion and oxidization, which comprises coating a ferrous workpiece with an aluminum alloy containing chromium in an amount within the range of 1 to 10 percent by weight based on the total weight of said aluminum alloy, heating the coated ferrous workpiece for at least 30 minutes at an elevated temperature of from 700* to 930*C, and heating the thus treated ferrous workpiece at a predetermined temperature within the range of 950* to 1,350*C which is lower than the melting point of said ferrous workpiece, under an oxidizing atmosphere for not more than 10 hours, thereby forming an alloy layer uniformly over the surface of said ferrous workpiece, said alloy layer containing iron and aluminum as its principle components and including a substantial layer of a Alpha -Al2O3 secured therein by the binding action of said iron, at the surface portion of said alloy layer, said surface portion containing chromium which acts as a binder for securing said substantial layer of Alpha -Al2O3 in cooperation with said iron.

5. A ferrous workpiece prepared according to the method of claim 1, having a surface portion formed with an alloy layer having relatively high resistances to corrosion and to oxidization, said alloy layer being 200 to 500 microns in depth and containing iron and aluminum as its principle components and including therein a substantial layer of Alpha -Al2O3 of 10 to 80 microns in depth secured in a surface portion of said alloy layer, and wherein at least said surface portion of said alloy layer contains chromium which acts as a binder for securing said substantial layer of Alpha -Al2O3 in cooperation with said iron.

6. A ferrous workpiece as claimed in claim 5, wherein said alloy layer includes a scattered Alpha -Al2O3 layer at a substantially intermediate portion of said alloy layer.