US3915756A

US3915756A - Process of manufacturing cast steel marine propellers

Info

Publication number: US3915756A
Application number: US295362A
Authority: US
Inventors: Teishiro Oda; Makoto Nakamura; Masato Zama
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1970-10-13
Filing date: 1972-10-05
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28

Abstract

A tenacious cast steel, is used for manufacturing propellers, and has essentially the following composition: C: between about 0.03-0.07% Cr: between about 10-14% Ni: between about 5-7% Co: between about 3-5% Mo: between about 1.0-2.5% Si: not more than about 0.8% Mn: not more than about 1.2% and IMPURITIES, THE BALANCE BEING Fe, all percentages being by weight on total composition. In the marine propellers, a portion of the nickel content of this cast steel may be replaced by copper. The copper amount, however, should not exceed 3% by weight and the ratio of copper to nickel should be about 1:1. The process produces martensite steels having therein precipitates of molybdenum compounds which reinforce the cast steel propellers improving ductility toughness over conventional cast steels.

Description

United States Patent Oda et al.

[ 1 Oct. 28, 1975 PROCESS OF MANUFACTURING CAST STEEL MARINE PROPELLERS [73] Assignee: Mitsubishi Jukogyo Kabushiki Kaisha, Japan [22] Filed: Oct. 5, 1972 [21] Appl. No.: 295,362

Related US. Application Data [63] Continuation-impart of Ser. No. 80,452, Oct. 13, 1970, abandoned, which is a continuation-impart of Ser. No. 884,452, Decv 12, 1969, abandoned.

[52] US. Cl 148/3; 75/125; 75/128 B; 75/128 W; 148/37 [51] Int. Cl. C2ld 9/00; C22c 39/26 [58] Field of Search 148/3, 37; 75/128 B [56] References Cited UNITED STATES PATENTS 2,499,860 3/1950 Hansen 148/3 2,557,971 6/1951 Jacklin 2,783,169 2/1957 Morgan..... 3,154,412 10/1964 Kasak 3,192,073 6/1965 .lominyr. 3,278,298 6/1974 Perry 3,650,845 3/1972 Ota 75/128 B 3,661,658 5/1972 Ota 148/3 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmMcG1ew & Tuttle [57] ABSTRACT A tenacious cast steel, is used for manufacturing propellers, and has essentially the following composition:

C: between about 0.03-0.07%

Cr: between about 10-14% Ni: between about 5-7% Co: between about 35% Mo: between about 1.02.5%

Si: not more than about 0.8%

Mn: not more than about 1.2% and impurities, the balance being Fe, all percentages being by weight on total composition.

In the marine propellers, a portion of the nickel content of this cast steel may be replaced by copper. The copper amount, however, should not exceed 3% by weight and the ratio of copper to nickel should be about 1:1. The process produces martensite steels having therein precipitates of molybdenum compounds which reinforce the cast steel propellers improving ductility toughness over conventional cast steels.

10 Claims, 2 Drawing Figures PROCESS OF MANUFACTURING CAST STEEL MARINE PROPELLERS REFERENCE TO PRIOR APPLICATIONS This is a continuation-in-part of pending application Ser. No. 80,452, filed on Oct. 13, 1970, which in turn is a continuation-in-part of application Ser. No. 884,452, filed on Dec. 12, 1969 both of which are now abandoned.

FIELD OF THE INVENTION The invention relates to manufacture of cast steel propeller constructions for marine use and to the cast steel marine propellers produced thereby.

BACKGROUND INFORMATION AND PRIOR ART In recent years, ships, such as tankers, have been constructed in ever increasing dimensions. This in turn has necessitated the construction of large size propellers or screws. The increased size of the propellers has, of course, resulted in heavier propellers, to wit, propellers of greater weight. The weight increase of a propeller in turn causes a number of difficult problems.' The heavier the propeller, the more material for its construction has to be used, which in turn increases the costs. Accordingly, the nature of the material to be used for large size propeller constructions is of considerable interest and emphasis has been placed on relatively inexpensive materials. Further factors, such as power loss for driving a large size propeller and difficulties in designing suitable stern structures for such propellers, have to be considered.

With the view to overcoming these inherent difficulties, attempts have been made to construct large size propellers from alloy steel compositions which are relatively lightweight, so as to decrease the weight of the final propeller, and which, at the same time, are relatively inexpensive while having satisfactory mechanical properties.

To this end, it has previously been suggested to construct large size propellers from tenacious or tough cast steel which essentially consists of not more than 0.25% of carbon, not more than 1.0% of silicon, not more than 3.0% of manganese, between about 5 to 20% of chromium, l to 8% of cobalt, 0.5 to 7% of molybdenum and/or tungsten, not more than 8% of nickel and/or not more than 4% of copper, the balance being iron. (Unless otherwise indicated in this specification, all parts and percentages are by weight, based upon the total composition.)

Propellers constructed from this prior art material and used under cathodic protecting conditions have satisfactory properties and performance characteristics, if the composition is gradually cooled after casting from the austenitizing temperature and if cooling generally is effected gradually after subsequent heat treatments. However, this prior art cast steel composition has the disadvantage that the ductility of the composition significantly decreases during the cooling of the cast steel from the austenitizing temperature at a cooling speed of 10C per minute or more. While this drawback perhaps is not fatal for a satisfactory performance of propellers constructed from this composition, it is highly undesirable, because the decrease in the ductility causes in turn a tendency 'for crack formation, particularly during welding operations. Thus, if parts made from such steel are heated to a temperature necessary for welding, which is required to connect different parts or for repair purposes, the crack formation tendency is a very important factor which can only be counteracted by considerably lengthening the cooling time after heat treatments.

SUMMARY OF THE INVENTION It is the primary object of the present invention to overcome the aforesaid disadvantages of the prior art methods of producing cast steel large sized propeller constructions having less desirable properties such as inferior ductility toughness. Another object is to provide a superior marine propeller of cast steel which is devoid of a tendency for crack formations.

A further object of the invention is to provide anovel cast steel marine propeller construction which has superior mechanical properties, is relatively inexpensive and of relatively low weight.

It is also an object of the invention to provide a process for making a steel composition for marine propellers which preserves its superior ductility characteristics without deterioration of other properties, even if the steel composition is cooled rapidly from the austenitizing temperature or after subsequent heat treatments.

A still further object of the invention is to provide a process for making a steel composition for marine propellers, whose superior ductility characteristics are independent from the cooling speed after the steel composition is subjected to heat treatments, for example, upon reheating to the austenitizing temperatureafter casting. I

Briefly, according to the invention, a tenacious cast steel propeller for marine use is manufactured with es-. sentially the following composition:

C between about 0.030.07%

Cr between about 1014% Ni between about 57% Co between about 3-5% Mo between about l.02.5%

Si not more than about 0.8% v

Mn not more than about 1.2% and impurities, the

balance being Fe, all of said percentages being by weight based on the total steel composition.

A portion of the nickel content in the aforesaid composition can advantageously be replaced by copper. The copper content, however, should not exceed 3% and the ratio of copper to nickel should then be about 1:1.

The cast steel of this invention is for the most part face-centered cubic austenite matrix; at sufficiently high temperatures, most of the matrix changes to bodycentered cubic ferrite or martensite, and yields precipitates of inter-metallic compounds consisting mainly of the inter-metallic compound of molybdenum, when the body-centered cubic lattice matrix is again heated to a temperature between 450 and 700C. The purpose of precipitating the inter-metallic compounds of molybdenum is that, if the cast steel is reinforced by theseintermetallic compounds, the ductility toughness characteristics of this cast steel are maintained up to a higher strength than those of conventional cast steels, e.g., those reinforced by carbides.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specifica tion. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawings:

FIG. 1 is a table indicating the chemical composition of cast steel test pieces A through F, while FIG. 2 is a table indicating the mechanical properties of the test pieces A through F of FIG. 1, after having been subjected to the various heat treatments indicated in the table.

The test pieces A through D were prepared by melting 30 kg of the respective alloy in a basic high frequency melting furnace and casting the composition in a sand mold. By contrast, test pieces E and F were prepared by melting 5 tons of the respective alloy in a basic Heroult furnace and then casting the material into ingots of 1 ton weight.

As can be seen from FIG. 2, the ductility of the test pieces B-l, C-1 and D-l, which were cooled rapidly after casting and were allowed to stand in cast condition, is low, while that of the test piece, which was reheated to the austenitizing temperature after casting, is much higher and independent from the cooling speed. The mechanical properties of the thus heat treated test pieces are thus improved if compared with those of known steels. It will be appreciated that although the test pieces represented in the tables have a specific composition embraced within the scope of percentage ranges mentioned hereinabove, other specific compositions may be used, provided they fall within the indi cated limit values.

The amount of impurities should be such as not to negatively affect the properties of the steel. The respective P and S contents should thus preferably not exceed 0.03%. The carbon content in the composition has a marked influence on the properties of the steel, particularly due to the simultaneous presence of nickel and copper. In a preferred embodiment, the carbon content should be about 0.05%, but excellent results are generally obtained if the indicated range of 0.3 to 0.07% is adhered to. As a general proposition, it is rather diffi cult exactly to control the carbon content in the composition.

The nickel content should be within the range of 57%. If the nickel content is too low, then the ductility cannot be satisfactorily maintained upon quenching the steel. On the other hand, if the nickel content is too high, the residual austenite content increases. As stated, a portion of the nickel content, however, may be replaced by copper, the latter however not to exceed 3%. This is so because, if the composition contains more than 3% of copper, then the stability of the steel is negatively affected. The best results are obtained if the copper content does not exceed 3% and the ratio of copper to nickel is about 1:1.

In respect to the chromium content, relatively high amounts are desirable, to wit, the chromium should be within the indicated ratio of to 14%. Chromium contents outside the indicated range are undesirable because, again, the stability of steel may then be affected. The chromium, however, similar to cobalt, has a lesser influence on the steel characteristics than have the carbon and nickel.

The cobalt content should be within the indicated range of 3 to 5%. If the cobalt content fluctuates too much, then the optimum ranges for the nickel and copper contents are affected.

Referring now to the molybdenum content, this may vary between the indicated range of 1.0 to 2.5%. The molybdenum significantly contributes to the strength characteristics of the material. If the molybdenum is significantly higher than 2.5%, then it is difficult to maintain the desired ductility by rapid cooling.

The silicon and manganese additions are required as the de-oxidizing elements in the steel making and may be incorporated in the indicated proportions of not more than 0.8 and 1.2%, respectively.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. Method of making marine propellers which comprises a. forming an alloy composition of tenacious cast steel comprising the following elements by weight: i. from about 0.03 to about 0.07% carbon,

ii. from about 10 to about 14% chromium,

iii. from about 5 to about 7% nickel,

iv. from about 3 to about 5% cobalt,

v. from about 1.0 to 2.5% molybdenum,

vi. not more than about 0.8% silicon,

vii. not more than about 1.2% manganese,

viii. substantially the balance being iron;

b. casting the aforesaid alloy composition in the form of a marine propeller construction;

0. reheating said cast marine propeller construction to the austenitizing temperature after casting; and

d. cooling the so-obtained marine propeller construction of substantially martensite steel having improved ductility, tensile strength and resistance to cracking at welding temperatures.

2. Method of making marine propellers, according to claim 1, which comprises replacing a portion of the nickel with copper, said copper amounting to not more than 3%, the ratio of copper to nickel being about 1:1.

3. Tenacious cast steel marine propeller formed from an alloy composition consisting essentially of the following elements by weight:

i. from about 0.03 to about 0.07% carbon,

ii. from about 10 to about 14% chromium,

iii. from about 5 to about 7% nickel,

iv. from about 3 to about 5% cobalt,

v. from about 1.0 to about 2.5% molybdenum,

vi. not more than about 0.8% silicon,

vii. not more than about 1.2% manganese,

viii. substantially the balance being iron; said alloy composition being cast in the form of a marine propeller construction, reheated to the austenitizing temperature after casting, and thereafter cooled, whereby a marine propeller construction is obtained having improved ductility, tensile strength and resistance to cracking at welding temperatures.

4. Tenacious cast steel propeller formed from an alloy composition, as set forth in claim 3 wherein a portion of nickel is replaced with copper, said copper amounting to not more than 3% by weight, the ratio of copper to nickel therein being about 1:1.

6 5. Marine propeller according to claim 4, in which 8. Marine propeller according to claim 4, in which said cast steel consists essentially of: said cast steel consists essentially of:

i. about 0.06% carbon, 1 i. about 0.06% carbon, ii. about 0.28% silicon, ii. about 0.34% silicon, iii. about 0.45% manganese, 5 iii. about 0.40% manganese,

iv. about 12.58% chromium,

v. about 1.91% molybdenum vi. about 4.62% nickel,

vii. about 4.21% cobalt, and

viii. about 1.85% copper and the balance iron. 10

6. Marine propeller according to claim 4, in which said cast steel consists essentially of:

i. about 0.03% carbon,

ii. about 0.23% silicon,

iii. about 0.29% manganese,

iv. about 0.12% chromium,

v. about 2.20% molybdenum,

iv. about 12.6% chromium,

v. about 1.96% molybdenum,

vi. about 5.57% nickel,

vii. about 3.93% cobalt, and

viii. about 1.62% copper, and the balance iron.

9. Marine propeller according to claim 3, in which said cast steel consists essentially of:

i. about 0.06% carbon,

ii. about 0.46% silicon,

iii. about 0.34% manganese,

iv. about 11.78% chromium,

about 455% nickel. v. about 1.90% molybdenum, vii. about 4.03% cobalt, and 1: about 591% mckeli and viii. about 1.95% copper, and the balance iron. abotft 415% cobalt and the balance 7. Marine propeller according to claim 4, in which Marine Propeller accorfimg to Clam whlch said cast steel consists essentially of: Said Cast steel conslsts essentlally of:

i. about 0.06% carbon, about 005% carbon. ii. about 0.17% silicon, about 046% Silicon, iii. about 0.50% manganese, about 037% manganese, iv. about 12.60% chromium, iv. about l2-0l% chromium, v. about 1.91% molybdenum, v. about 1.88% molybdenum, vi. about 5.55% nickel, vi. about 5.94% nickel, and vii. about 4.48% cobalt, and vii. about 4.12% cobalt, and the balance iron. viii. about 1.90% copper, and the balance iron. 3

Claims

1. METHOD OF MAKING MARINE PROPELLERS WHICH COMPRISES A. FORMING AN ALLOY COMPOSITION OF TENACIOUS CAST STEEL COMPRISING THE FOLLOWING ELEMENTS BY WEIGHT: I. FROM ABOUT 0.03 TO ABOUT 0.07% CARBON II. FROM ABOUT 10 TO ABOUT 14% CHROMIUM, III. FROM ABOUT 5 TO ABOUT 7% NICKEL, IV. FROM ABOUT 3 TO ABOUT 5% COBALT, V. FROM ABOUT 1.0 TO 2.5% MOLYBDENUM, VI. NOT MORE THAN ABOUT 0.8% SILICON, VII. NOT MORE THAN ABOUT 1.2% MANGANESE, VIII. SUBSTANTIALLY THE BALANCE BEING IRON, B. CASTING THE AFORESAID ALLOY COMPOSITION IN THE FORM OF A MARINE PROPELLER CONSTRUCTION, C. REHEATING SAID CAST MARINE PROPELLER CONSTRUCTION TO THE AUSTENITIZING TEMPERATURE AFTER CASTING, AND D. COOLING THE SO-OBTAINED MARINE PROPELLER CONSTRUCTION OF SUBSTANTIALLY MARTENSITE STEEL HAVING IMPROVED DUCTILITY, TENSIL STRENGTH AND RESISTANCE TO CRACKING AT WELDING TEMPERATURES,

3. TENACIOUS CAST STEEL MARINE PROPELLER FORMED FROM AN ALLOY COMPOSITION CONSISTING ESSENTIALLY OF THE FOLLOWING ELEMENTS BY WEIGHT: I. FROM ABOUT 0.03 TO ABOUT 0.07% CARBON, II. FROM ABOUT 10 TO ABOUT 14% CHROMIUM, III. FROM ABOUT 5 TO ABOUT 7% NICKEL, IV. FROM ABOUT 3 TO ABOUT 5% COBALT, V. FROM ABOUT 1.0 TO ABOUT 2.5% MOLYBDENUM, VI. NOT MORE THAN ABOUT 0.8% SILICON, VII. NOT MORE THAN ABOUT 1.2% MANGANESE, VIII. SUBSTANTIALLY THE BALANCE BEING IRON, SAID ALLOY COMPOSITION BEING CAST IN THE FORM OF A MARINE PROPELLER CONSTRUCTION, REHEATED TO THE AUSTENITIZING TEMPERATURE AFTER CASTING, AND THEREAFTER COOLED, WHEREBY A MARINE PROPELLER CONSTRUCTION IS OBTAINED HAVING IMPROVED DUCTILITY, TENSILE STRENGTH AND RESISTANCE TO CRACKING AT WELDING TEMPERATURES.

5. Marine propeller according to claim 4, in which said cast steel consists essentially of: i. about 0.06% carbon, l ii. about 0.28% silicon, iii. about 0.45% manganese, iv. about 12.58% chromium, v. about 1.91% molybdenum vi. about 4.62% nickel, vii. about 4.21% cobalt, and viii. about 1.85% copper and the balance iron.

6. Marine propeller according to claim 4, in which said cast steel consists essentially of: i. about 0.03% carbon, ii. about 0.23% silicon, iii. about 0.29% manganese, iv. about 0.12% chromium, v. about 2.20% molybdenum, vi. about 4.55% nickel; vii. about 4.03% cobalt, and viii. about 1.95% copper, and the balance iron.

7. Marine propeller according to claim 4, in which said cast steel consists essentially of: i. about 0.06% carbon, ii. about 0.17% silicon, iii. about 0.50% manganese, iv. about 12.60% chromium, v. about 1.91% molybdenum, vi. about 5.55% nickel, vii. about 4.48% cobalt, and viii. about 1.90% copper, and the balance iron.

8. Marine propeller according to claim 4, in which said cast steel consists essentially of: i. about 0.06% carbon, ii. about 0.34% silicon, iii. about 0.40% manganese, iv. about 12.6% chromium, v. about 1.96% molybdenum, vi. about 5.57% nickel, vii. about 3.93% cobalt, and viii. about 1.62% copper, and the balance iron.

9. Marine propeller according to claim 3, in which said cast steel consists essentially of: i. about 0.06% carbon, ii. about 0.46% silicon, iii. about 0.34% manganese, iv. about 11.78% chromium, v. about 1.90% molybdenum, vi. about 5.91% nickel, and vii. about 4.15% cobalt, and the balance iron.

10. Marine propeller according to claim 3, in which said cast steel consists essentially of: i. about 0.05% carbon, ii. about 0.46% silicon, iii. about 0.37% manganese, iv. about 12.01% chromium, v. about 1.88% molybdenum, vi. about 5.94% nickel, and vii. about 4.12% cobalt, and the balance iron.