US3926622A

US3926622A - Pitting resisting alloy steels

Info

Publication number: US3926622A
Application number: US510542A
Authority: US
Inventors: Kingo Kiyonaga; Yoshiyuki Murakawa
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 1973-10-03
Filing date: 1974-09-30
Publication date: 1975-12-16
Anticipated expiration: 1992-12-16
Also published as: SE7412392L; GB1481875A; SE399913B; DE2447137B2; JPS5611745B2; JPS5061317A; DE2447137A1

Abstract

Pitting resisting alloy steels in which resistance to pitting is increased without adversely affecting superb mechanical properties and impairing the character of the microstructure of high-chromium alloy steels by adding cerium thereto. The highchromium alloy steels comprise carbon and chromium in ratios by weight which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification and showing carbon-chromium ratios, and are added with cerium. In addition to cerium, molybdenum, tungsten, vanadium, cobalt, nickel and/or copper may be added in suitable proportions to the high-chromium alloy steels if it is desired to increase their wear resistance, strength at elevated temperatures and corrosion resistance.

Description

United States Patent [1 1 Kiyonaga et al.

[ Dec. 16, 1975 [54] PITTING RESISTING ALLOY STEELS [73] Assignee: Hitachi Metals, Ltd., Japan 22 Filed: Sept. 30, 1974 [21] Appl. No.: 510,542

[30] Foreign Application Priority Data Oct. 3, 1973 Japan 48-110512 [52] US. Cl. 75/126 G; 75/126 C; 75/126 E; 75/126 H; 75/128 B; 75/128 E; 75/128 V; 75/75; 75/128 W [51] Int. Cl. C22C 38/18 [58] Field 01' Search 75/126 G, 126 C, 126 E, 75/126 H, 128 E, 128 W, 128 B, 128 V [56] References Cited UNITED STATES PATENTS 3,746,536 7/1973 Kuse 75/126 D X 3,876,475 4/1975 Ramqvist 75/128 E X Primary Examiner-L. Dewayne RUllCLl Assistant ExaminerArthur J. Steiner Attorney, Agent, or FirmCraig & Antonelli [5 7] ABSTRACT Pitting resisting alloy steels in which resistance to pitting is increased without adversely affecting superb mechanical properties and impairing the character of the microstructure of high-chromium alloy steels by adding cerium thereto. The high-chromium alloy steels comprise carbon and chromium in ratios by weight which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification and showing carbon-chromium ratios, and are added with cerium. In addition to cerium, molybdenum, tungsten, vanadium, cobalt, nickel and/or cop per may be added in suitable proportions to the high chromium alloy steels if it is desired to increase their wear resistance, strength at elevated temperatures and corrosion resistance.

12 Claims, 1 Drawing Figure PITTING RESISTING ALLOY STEELS This invention relates to pitting resisting highchromium alloy steels added with cerium.

It is known that high-chromium alloy steels are highly corrosion resistant and wear resistant. Because of these properties, they are used widely for producing edges tools, valve seats component parts of pumps and the like. The high corrosion resistance of high-chromium alloy steels is attributed to the formation on their surfaces of a coat of high consistency of the oxides of chromium whereby the steels are brought to a passive state. The metals which have the tendency of being This invention has as its object the provision of pitting resisting alloy steels which are high-chromium alloy steels of high strength and high wear resistance which contain more than 0.3 carbon. The outstanding characteristic of the invention is that resistance to pitting of such high-chromium alloy stvels can be increased by adding a small amount of cerium thereto, without adversely affecting the properties of such steels.

The drawing is a graph showing the range to which the carbon and chromium contents of the alloy steels are limited.

Table 1 shows the chemical compositions of alloy steels according to the invention in comparison with brought to a passive state have the disadvantage of 15 those of steels of the prior art.

TABLE 1 Sample C Si Mn P S Cr Mo Ce The others Remarks 1 0.98 1.03 0.41 0.014 0.003 6.16 0.1 Steels Accod ing to the Invention 2 0.83 0.95 0.43 0.013 0.004 9.50 1.08 0.25 3 1.47 0.57 0.40 0.013 0.002 11.03 0.86 1.5 4 0.36 0.97 0.55 0.015 0.004 5.50 0.5 Co=1.03 5 2.25 1.03 0.60 0.012 0.003 19.32 0.5 6 1.05 1.12 0.48 0.012 0.003 6.26 0.5 V=0.28

Ni=0.58 7 0.70 1.13 0.50 0.014 0.003 10.30 0.94 0.25 Cu=0.30 8 0.87 0.86 0.47 0.018 0.005 11.84 1.04 0.15 V=0.63

Ni=0.84 9 0.46 0.60 1.48 0.011 0.006 13.49 0.50 W=1.83

Co=1.04 V=0.54 10 0.74 0.96 0.62 0.014 0.003 9.68 0.75 11 1.02 1.15 0.53 0.012 0.008 6.21 Steels of the Prior Art 12 0.73 1.08 0.52 0.013 0.009 10.12 1.10 13 0.64 0.44 0.58 0.011 0.007 13.29 14 1.44 0.60 0.73 0.012 0.008 11.20 0.77 V=0.24 15 0.72 1.14 0.61 0.014 0.010 12.08 0.96 Co=1.90

Note:

Cerium is expressed in proportions (7:) added to the alloy steels while other elements in proportions (7c) contained therein.

developing pitting when the passive state is interrupted and the coat is broken locally for some reason. As a result, pits are formed at minuscule points on the surfaces of the metals because they are activated is spite of the fact that the major portion of the surfaces of the metals is in the passive state.

In the case of ordinary chromium steels, pitting tends to occur in steels of the alloy structure in which the coat formed for bringing about a passive state is imperfect. Generally, steels of high chromium content and low carbon content are readily brought to a passive state and the coat formed is stable because it is of high consistency, so that pitting seldom occurs. Steels of low carbon content are not fit to produce products which must have wear resistance as well as resistance to pitting, because their hardenability by heat treatment and wear resistance are reduced. On the hand, highchromium alloy steels of high carbon content have the disadvantage of pitting occurring therein due to the fact that the coat formed to bring the steels to a passive state is unstable.

Table 2 shows the results of tests conducted on the steels shown in FIG. 1 for hardness and resistance to pitting after the steels were quenched and tempered. The specimens used for the tests of Table 2 were annealed pieces of 6 mm X 15 mm X 40 mm. Quenching consisted of oil quenching effected after the specimens were maintained at various temperatures for three minutes, and a subzero treatment performed at 78C for ten minutes. In tempering the specimens, they were heated for one hour. After the heat treatment, the thickness of each specimen was reduced by about one millimeter by grinding one side only and then hardness tests were carried out. In subjecting the specimens to pitting resistance tests, the surfaces of the specimens were finished by using Emery grinding paper 500 and fat was removed therefrom. Then, the specimens were immersed in tap water and an aqueous solution of 0.1 N-Nacl for twelve hours. After lapse of the predetermined length of time, the number of pits formed in an area 13 X 38 square millimeters was counted in each specimen for comparison.

TABLE 2 Samples Quenching Tempering Hardness Fitting Resistance Remarks (C) (C) (Hv) Tap Water 0.1N-Nacl l 1050 300 772 l 2 Steels According to the Invention 2 732 O [1 3 200 764 0 2 4 550 579 0 0 5 200 760 l 2 6 300 754 1 l TABLE 2-continued Samples Ouenching Tcmpering Hardness Pitting Resistance Remarks (C) (C) (Hv) Tap Water 0.lN-Nacl lO 1075 686 O 0 ll 1050 300 770 l3 l Steels of the Prior Art 13 l l00 650 0 l As can be seen in Table 2, steels of higher chromiumcarbon ratios are higher in pitting resistance but lower in quench and temper hardness than steels of lower chromium-carbon ratios. It will be seen that the addition of cerium exerts no influences on the hardening of the alloy steels by heat treatment. However, comparison of the alloy steels according to the invention with those of the prior art shows clearly that the addition of cerium markedly increases the resistance of the alloy steels to pitting, provided that the chromium-carbon ratios of the steels are of the same degree.

The reason why resistance of high-chromium alloy steels to pitting can be increased by adding a small proportion of cerium to them is considered to be as follows. The results of tests conducted many times have shown that inclusions of the sulfide base, MnS in particular, entrapped in the molten metal tend to become activated locally and form the nuclei for forming pits. On the other hand, cerium is considered to more readily react with sulphur than manganese and perform a high desulfurizing action, so that cerium will have the effect of precluding pitting by acting in the form of Ce2S3 or Ce3S4. The aforementioned description is considered to explain the mechanism of inhibiting pitting by the addition of cerium.

The reason why limitations are placed on the chemical compositions of the alloy steels according to the invention will now be described. The alloy steels according to the invention contain carbon in order to impart to the steels sufficiently high hardness and strength to enable the products made of the steels to have a long service life. Thus, the carbon content should be at least over 0.3 or preferably over 0.5 However, when the carbon content is over 2.3 the hot workability of the steels is markedly reduced and difficulty is encountered in producing desired products from them. Therefore, the proportion of carbon contained in the alloy steels according to the invention is limited to below 2.3

Chromium increases the corrosion resistance of a steel and particularly it renders a steel anntipitting. When the chromium content of a steel is below 5 the steel is very low in pitting resistance and cannot serve as an anticorrosion alloy. On the other hand, an increase in resistance to pitting cannot be expected even if the chromium content is increased to a level over By considering the matter from the economical point of view, the chromiumceliitent is limited to a range from 5 to 20 preferably from 8 to 18 in the alloy steels according to the invention. Generally speaking, resistance to pittiilg Varies from steel to steel depending on the chronill'ilii earbon ratio; the greater the ratio, the higher the Blitllig resistance.

Generally, when the chromium-carbon ratio of a steel is over 20, its resistance to pitting is quite satisfactory. This makes it unnecessary to increase resistance to pitting by adding cerium to a steel as is done in the present invention. However, when a steel has a chromium-carbon ratio of below 5, its resistance to pitting is greatly reduced because of the paucity of chromium in the base. In such case, addition of cerium does not lead to improved characteristics. In order that the addition of cerium may achieve satisfactory results in increasing resistance to pitting, steels to which cerium is added should have a chromium-carbon ratio which is below 20 and above 5 or preferably below 20 and above 8. By taking the aforementioned observations into consideration, the contents of carbon and chromium of alloy steels according to the invention are limited to a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A or preferably A, B, C, D, E and A in the drawing accompanying the specification. Silicon and manganese are contained in ordinary stainless steels. If their contents are below 2 it will adversely affect the workability of the steels, so that their contents are limited to below 2.0

Theoretically, the addition of cerium would produce satisfactory results in increasing their resistance to pitting in other types of chromium, base alloy steels than the type described above. Thus, when it is desired to increase the wear resistance and the strength at elevated temperatures of alloy steels, one or more than two elements selected from among the group consisting of molybdenum, tungsten, vanadium and cobalt may be added. Also, when it is desired to further increase corrosion resistance, nickel and/or copper may be added singly or in combination. The addition of these elements to the alloy steels does not in any way affect the satisfactory results which the addition of cerium to them can achieve in increasing their resistance to pitting. Molybdenum, tungsten, vanadium and cobalt are elements which are effective to minimize a reduction in hardness which would otherwise be caused by tempering and at the same time to increase strength at elevated temperatures. The upper limit of the proportion of each of these elements is limited for economical reasons to 2.5 each when added singly and to 4.0 in total when added in combination. Nickel and copper have a strong influence on the formation of an austenite, so that their contents are each limited to below 2.5 Besides, the addition of cerium to the type of highchromium alloy steels containing titanium, zirconium, boron and niobium in order to increase strength in hot working can achieve the same results in increasing resistance to pitting as those achieved in the previously described types of high-chromium alloy steels.

, If the addition of cerium were to achieve satisfactory results, its proportion .would have to be over 0.01

preferably over 0.05 However, the addition of cerium in proportions which are too high degrades the purity of steels when they are produced and renders the ingots hard to forge. Thus, the proportion in which cerium is added to highchromium alloy steels is limited to a range from 0.01 to 2.0 preferably from 0.05 to 0.5 according to the invention. When cerium is added, it combines with sulphur and oxygen (0 and removed from of the steel as slag. Therefore, in actual practice, the amount of cerium added to alloy steels is more important than the amount ,of cerium contained in them. Thus, the amount of cerium is expressed in a proportion in which it is added to the alloy steels, rather than in a proportion in which it is contained therein. Economically, it is preferable to add cerium in the form of mother alloy which contains cerium in several scores of percents. When this is the case, however, rare earth elements, e.g. lanthanum, neodymium,

praceodymium, and Samarium, or magnesium, aluminum and other metals will be contained in the alloy steels, through small in proportion.

Cerium readily reacts with oxygen (0 In view of this phenomenon, it will be advantageous to use a suitable deoxidizer, such for example as Fe-Si, Ca-Si or Al, simultaneously as cerium is added, or to add cerium after deoxidizing is effected beforehand. When this is the case, calcium, aluminum and other elements will be contained in the alloy steels. However, these elements can be contained as impurities in the alloy steels according to the invention.

This invention has particular utility when applied to high-carbon high-chromium alloy steels comprising carbon in a range from 0.6 to 1.0 and chromium in a range from 8 to 13 Quenching and tempering impart a high degree of hardness to these steels which are also highly wear resisting. These properties make these steels fit for producing edged tools of stainless steel. However, the edged tools made of stainless steel tend to develop pitting due to circumstances under which they are put to use. Since the addition of cerium to the aforementioned alloy steels can prevent the occurrence of pitting, these steels have a practical value for producing edged tools of stainless steel in the true sense of the words.

We claim:

1. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, and the balance iron and impurities.

2. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprised, and the balance iron and impurities.

3. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together po ints'of, coordinates A, B, C, D, E and .A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprised, less than 2.5 nickel, less than 2.5 copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and'impurities.

' 4. 'Pitting'resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D", E and A in the drawing accompanying the specific'ation, less than 2.0 %'silicon, less than 2.0% manganese, and the balance iron and impurities.

5. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprised, and the balance iron and impurities.

6. Pittin g resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprised, less than 2.5 nickel, less than 2.5 copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and impurities.

7. Pitting resisting alloy steels consisting essentially of by weight 0.05 to 0.5 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, and the balance iron and impurities.

8. Fitting resisting alloy steels consisting essentially of by weight 0.05 to 0.5 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, one or more two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprised, and the balance iron and impurities.

9. Pittin g resisting alloy steels consisting essentially of by weight 0.05 to 0.5 cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C,

D, E and A in the drawing accompanying the specification, less than 2.0 silicon, less than 2.0 manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprises, less than 2.5 nickel, less than 2.5 copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and impurities.

l0. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, 0.6 to 1.0 carbon, 8.0 to 13.0 chromium, less than 2.0 silicon, less than 2.0% manganese, and the balance iron and impurities.

l1. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, 0.6 to 1.0 carbon, 8.0 to 13.0 chromium, less than 2.0 silicon, less than 2.0 manganese, one or more than two elements 8 selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprised, and the balance iron and impurities.

12. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 cerium, 0.6 to 1.0 carbon, 8.0 to 13.0 chromium, less than 2.0 silicon, less than 2.0 manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 each when one of them is comprised and in less than 4.0 in total when more than two of them are comprised, less than 2.5 nickel, less than 2.5 copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and impurities.

UNITED STATES PATENT OFFICE QETIFICATE OF CORRECTIGN Patent NO. Dated DEC. 6,

I KIYONAGA ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title Page, column 2, lines 8 and 9 of the Abstract, change 'coordinates A, B, C, D, E and A" to coordinates A, B, C, D, E, F and'A Column 4, line 33, change coordinates A, B, C, D, E and A" to coordinates A, B, C, D, E, F and A Claim 1, lines 4 and 5, change "coordinates A, B, c, D, E and A" to coordinates A, B, C, D, E, F andA Claim 2, lines 4 and 5, change "coordinates A, B, C, D, E, and A" to coordinates A, B, C, D, E, F, and A Claim 3, lines 4 and 5, change coordinates A, B, C, D, E and A" to coordinates A, B, C, D, E, F and A Signed and Scaled this twenty-seventh Day Of April1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (onmzissium'r uflatz'nls and Trademarkx

Claims

1. Pitting resIsting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, and the balance iron and impurities.

2. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprised, and the balance iron and impurities.

3. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A, B, C, D, E and A in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprised, less than 2.5 % nickel, less than 2.5 % copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and impurities.

4. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A'', B'', C'', D'', E'' and A'' in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, and the balance iron and impurities.

5. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A'', B'', C'', D'', E'' and A'' in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprised, and the balance iron and impurities.

6. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A'', B'', C'', D'', E'' and A'' in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprised, less than 2.5 % nickel, less than 2.5 % copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and impurities.

7. Pitting resisting alloy steels consisting essentially of by weight 0.05 to 0.5 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A'', B'', C'', D'', E'' and A'' in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, and the balance iron and impurities.

8. Pitting resisting alloy steels consisting essentially of by weight 0.05 to 0.5 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A'', B'', CD'', E'' and A'' in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, one or more two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprised, and the balance iron and impurities.

9. Pitting resisting alloy steels consisting essentially of by weight 0.05 to 0.5 % cerium, carbon and chromium in ratios which are in a region bounded by straight lines connecting together points of coordinates A'', B'', C'', D'', E'' and A'' in the drawing accompanying the specification, less than 2.0 % silicon, less than 2.0 % manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprises, less than 2.5 % nickel, less than 2.5 % copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and impurities.

10. PITTING RESISTING ALLOY STEELS CONSISTING ESSENTIALLY OF BY WEIGHT 0.01 TO 2.0 % CERIUM, 0.6 TO 1.0 % CARBON, 8.0 TO 13.0 % CHROMIUM, LESS THAN 2.0 % SILICON, LESS THAN 2.0 % MANGANESE, AND THE BALANCE IRON AND IMPURITIES.

11. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, 0.6 to 1.0 % carbon, 8.0 to 13.0 % chromium, less than 2.0 % silicon, less than 2.0 % manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprised, and the balance iron and impurities.

12. Pitting resisting alloy steels consisting essentially of by weight 0.01 to 2.0 % cerium, 0.6 to 1.0 % carbon, 8.0 to 13.0 % chromium, less than 2.0 % silicon, less than 2.0 % manganese, one or more than two elements selected from the group consisting of molybdenum, tungsten, vanadium and cobalt, said elements being in less than 2.5 % each when one of them is comprised and in less than 4.0 % in total when more than two of them are comprised, less than 2.5 % nickel, less than 2.5 % copper, said nickel and said copper being comprised either singly or in combination, and the balance iron and impurities.