US3386862A - High strength structural steel - Google Patents

Description

June 4, 1968 T. L. JOHNSTON ET AL HIGH STRENGTH STRUCTURAL STEEL Filed June 22, 1966 2 Sheets-Sheet 1 FIGIZ RAYMOND C. 505 T TNER RICHARD H. BUSH THOMAS L. JOHNSTON ARTHUR J-MEV/LKJR.

/N VENTORS 4TTORNEVS United States Patent Ofice 3,386,862 Patented June 4, 1968 3,386,862 HIGH STRENGTH STRUCTURAL STEEL Thomas L. Johnston, Bloomfield Hills, Raymond C. Boettner, Dearborn Heights, Richard H. Bush, Detroit, and Arthur J. McEvily, Jr., Dearborn, Mich., assignors to Ford Motor Company, Dearborn, Mich, a corporation of Delaware Continuation-impart of application Ser. No. 459,320,

May 27, 1965. This application June 22, 1966, Ser.

6 Claims. (Cl. 148-12) This application is a continuation-in-part of application Ser. No. 459,320 filed May 27, 1965, and assigned to the assignees of this application and now abandoned.

This invention is concerned with a high strength structural steel and the method by which this steel is produced. This high strength structural steel is weldable and in the as-rolled condition possesses a minimum yield strength of 80 K.s.i. and an impact energy absorption at -40 F. of 40 foot pounds. This alloy is economical of expensive alloying ingredients and may be prepared either by air melting or vacuum melting techniques. No particular rolling procedures are required.

The unique properties of these high strength structural steels as compared to ordinary low alloy structural steel are readily apparent from a study of the three figures of drawing in which FIGURE 1 is an electron beam transmission micro graph of the high strength structural steel of this invention. The structure is a massive martensite which is essentially ferrite containing a high density of dislocations. The magnification employed is X 20,000. The chemistry of this sample is:

Percent Carbon 0.02 Nickel 3.00 Molybdenum 3.00 Manganese 0.70 Silicon 0.30 Niobium 0.05

Remainder essentially all iron.

In this alloy both nickel and manganese serve to depress the austenite to massive martensite transformation temperature, while molybdenum serves to inhibit the transformation of austenite to equiaxed ferrite and pearlite prior to the transformation of austenite to massive martensite. The molybdenum may also serve as a carbide former. Chromium may be substituted in part for the more expensive molybdenum. To be commercially acceptable, each alloy should contain sufficient nickel or manganese or a combination of both metals to provide an acceptable austenite to massive martensite transformation temperature. The alloy should also contain a carbide forming metal which should be either chromium or molybdenum or a combination of these two metals which serves to inhibit the transformation of austenite to equiaxed ferrite and pearlite prior to the transformation temperature.

FIGURE 2 is a comparable electron beam transmission micrograph of a low alloy structural steel having a like thermal history. The structure is equiaxed ferrite containing a low density of dislocations. The magnification employed is x 20,000. The nominal chemistry of this alloy Percent Carbon 0.02 Nickel 3.00 Silicon 0.30 Manganese 0.70

Remainder essentially all iron.

In contrast to heat treated alloy steels containing in excess of 0.1% carbon which derive their strength from quenched and tempered martensite or lower bainite, we have found that comparable yield strengths together with excellent impact toughness and weldability in the as-rolled condition can be obtained in very low carbon massive martensite steels. The carbon content of these steels should not exceed 0.07%. These steels are alloyed so as to transform on air cooling to massive martensite. The carbon content is minimized, and the alloying elements are selected to avoid the transformation to equiaxed ferrite during rolling or cooling. The austenite should be stable during processing. On subsequent cooling, the alloy should transform from austenite to massive martensite. The temperature at which this transformation takes place should not exceed 1200 F., and the preferred range is 960 to 1150 F. In general the yield strength varies inversely as the austenite to massive martensite transformation temperature.

The strength properties of air cooled plates are relatively insensitive to rolling temperatures below the austenite grain coarsening range at about 1700 to 1800 F. Impact properties are sensitive to the prior austenite grain size because the latter controls the density of grain boundary impurities. This dictates that excessive soaking times are to be avoided after completion of rolling at temperatures above 1700 F.

All heats were of one size, 20 lbs., with the exception of one 40 lb. heat, and were made in induction furnaces either in air or in vacuo. In air melts, chromium, manganese, silicon, molybdenum and carbon were added as ferro alloys. In vacuum melts, all alloying additions were in elemental form.

In air melts, after the melt down of the base charge of (Armco) iron, electrolytic nickel, and ferro-molybdenum, an addition of 0.05% aluminum was made. This addition was followed by ferro additions of chromium, manganese, silicon and vanadium, carbon and pure niobium as required. Prior to pouring at 2800-2850 R, an additional 0.05% aluminum was added.

In vacuum melts, the starting charge was electrolytic iron and electrolytic nickel. After melt down in a vacuum of the order of 0.1 mm. Hg, an argon atmosphere at a pressure of 500 mm. Hg was introduced prior to the addition of manganese and silicon. Manganese has a high vapor pressure, and the presence of the argon atmosphere reduces its loss from the melt due to vaporization. The argon atmosphere was maintained while further additions of other desired elements were made and the melt poured.

For both air and vacuum melts, the ingots were cast in tapered chill molds with a mean diameter of 2 inch and were allowed to cool in the molds at room temperature. After removal from the molds the ingots were sectioned transversely to provide two-ten pound cylinders for rolling. Both visual and X-ray examination of selected ingots indicated them to be essentially free of casting defects.

ROLLING PROCEDURE The usual rolling practice consisted of annealing the ingot halves at 1775 F. for 40 minutes after which they were box-rolled to 2 inch square bars using flat rolls with /s inch reductions in roll opening and 5 minute reheats at 1775 F. after alternate passes. The bar temperature was not allowed to drop below 1400 in this stage. After box-rolling, the square bar was rolled to inch plate either in the temperature range of 1700-1775 F. with intermediate reheats, or was air cooled to 1400 F. and rolled without reheats. The finishing temperature in the latter case was approximately 1350" F. In both rolling sequences the resultant 2 x 14 inch plate was air cooledto room temperature.

yield strength, ten- Charp bars. However, because of the large number of alloys investigated, complete transition curves were not always obtained. For purposes of screening, a single test at 40 F. was often used. 5 Certain of the alloys which were produced in the investigation of this class of alloys are defined below in Table 1. In this table V indicates a vacuum melt and A an air melt.

TABLE 1 Alloy Compositions, w/o

Nb Al Ti The mechanical properties such as percent elongation and the reduction in area The impact tests were made on standard 45-V-notch m m 28101082080401 w 23923257470430 h 22 11 111 C t n 1 6799 23 5477 RR 7Wr0550$60$4550 e p t n e M 22122111221111 8 11111111111111 p UL 91970142532842 34333333554444 11111111111111 K S 3 6333 9355451 7 0U1111m0111111 Y2 11111111111111 nm e O 1 .U m 00000000 00 9 .ma 05050505 0 F 47474747 47 D. P 1 1 11 LLL1 1 1 1 m m 6 C 1 2 T a 0 E .1 1 I 11 n .1 6 L m B u u n n n t m m A vw n u n u T 2 an e n n n u h S e V o M W m u u h 1 1 :0 002 010152013 1 a l 9000 000002210 w m A u u u 23323323333333 C a n v n a u h 2 n mm H n n u M 0 H t u 00 S 77773 765 H v n u H k u 2 a2 22222222222 w m 0 m H u u u 2 S n m i m m m m n 50 0 0 O a n u 0 5 0 5055 5 i t mm%%- &oo%fimmmnom7mmfi 9059go H 0 u u M t :[v u u n u U 2 1LL13333 n m d C u u n 2222 r J h" n 0 6 m u N n n t m r. H Sv n a O y 05 5.0050050555505080 5 0550 .%%98%88998888999990701m1 1090 n u m u n n u 533143333 u T m h n u h 2222 n n m m o n n n H m m m u n w W 3 W n u u n 0 t b r 1 a e H u n n u 0 e S m. n u 2 r a n m h e e n u n u h t m t .m n n W 2 g m u n n e n n n n .1 N 0 mn u 9 t 9 0 5 T a O a U m a 3 w 4 0 5 1 T. 8 1 1 2 2 2 2 2 2 T H 2 2 2 2 2 2 2 Z f Z Z Z Z Z Z Z A O V V A A V A A sile strength were determined on the basis of tensile tests of /8 inch diameter specimens. The elongation values were determined over a .8 inch gage length (7D), rather than over a usual 4D gage length. Hence the values obtained are about two-thirds that expected for a value measured over the smaller gage length.

Heat Number TABLE 2Gontinued C, Si, Nb, Al, V, Rolling Y.S. T.S. R.A., Charpy Heat Number w/o w/o w/o w/o w/o Tempei ature, (1.%%) K s i 6, percent percent (71015.),

AZ2261 l, 400 96 127 12 59 59 1, 750 99 124 13 70 AZ2262 1, 400 111 143 11 58 58 1, 750 110 138 11 58 64 AZ2312 1, 400 120 155 11 50 48 1,750 112 152 10 41 AZ2313 1, 400 103 139 12 48 58 1, 750 100 139 13 50 72 27 1,400 67 81 19 71 239 27 1,400 110 135 10 58 17 29 1,400 118 141 9 54 11 1,700 112 136 11 60 10 32 .055 040 19 1,400 101 116 12 94 1, 700 99 115 11 74 116 24 055 045 17 1,400 106 124 11 64 115 1, 700 105 124 11 74 28 060 035 .I 1,400 123 11 70 48 I 1,700 100 117 11 75 100 31 057 16 l, 400 112 131 10 67 20 1, 700 10 71 95 .3 1 1 .1 1,400 113 135 10 71 19 1, 700 113 132 10 73 5 42 17 1,400 118 142 10 64 16 1,700 113 135 10 72 11 34 035 1,400 107 128 11 73 41 1,700 102 121 11 75 15 29 065 1, 400 117 149 12 67 135 1, 700 114 143 11 72 150 27 05 1, 400 143 176 10 54 13 1, 700 136 171 ll 59 14 27 05 04 01 1, 400 140 176 10 53 21 1, 700 132 ll 49 8 21 03 03 19 1, 400 140 178 10 36 14 1, 700 135 171 11 50 18 14 O3 01 .18 1,400 141 167 ll 55 17, 30,14

1, 700 131 165 11 61 39, 21, 10 25 060 19 1,400 143 180 10 60 17 e 1,700 134 173 10 61 25 22 19 1,400 133 175 11 62 22 1, 700 132 172 11 65 13 O8 040 040 13 1,400 130 169 10 64 36 1, 700 128 167 11 64 30 AZ2476 a 1 .05 14 055 .048 1, 4 8g 1%1 17g 27 1 9 17 11 64 30 1 Nominal composition. 1 .5 Mn addition. Table 3 has been prepared to illustrate the effect of cooling at different rates from 1400 F.

microscopy is seen to consist of ferrite plates containing a high density of dislocations. Such a configuration of TABLE 3 [Alloy (V Z2052).0032C-2.95Ni-2.75Mo] Cooling Rate Y.S. 'I.S., e R.A., Char F. mlnr Cooling Procedure (2%), K s.i. Percent Percent (-40 Y),

K 5.1. (tn-lb.

1,400 Estimated for water quench 100 109 12 77 238 following rolling.

100 Avefiage for air cooling following 99 109 14 79 151 ro ing.

6 hr. at 1775" F. followed by 94 99 12 79 24 furnace cool at constant rate.

2% Furnace cool following rolling 69 89 24 78 239 The results of tests of determine the effect of the rolling temperature on the mechanical properties are contained in Table 2. Below 1750 F. down to 1350 F., a range in which the alloys are austenitic, there appears to be little effect of the rolling temperature on the properties. For both the Mo-Ni alloy and the Cr-Mn alloy, yield strengths in excess of 100,000 p.s.i. are obtained. The impact values are higher in vacuum melts than in air melts, but in both cases the toughness is high as shown in FIGURE 3 where some of the present results are plotted and compared with the impact properties of T-1, 9Ni, HY-80, (9) 5Ni-Cr-Mo-V, and 1Mn steels. The reduction in area of the low carbon alloys i high, a direct indication that particle effects in ductile fracture are small. In addition, a high reduction in area has often been correlated with a high value of the shelf energy.

The transformation product observed in both the 3Mo-3Ni and the 3Cr-2.5Mn alloys was predominantly massive martensite. The leaner alloys start to transform at higher temperatures and they contain a much greater proportion of equiaxed ferrite. Whenever equiaxed ferrite formed in large amounts, the transformation range was not only higher, but also much broader than that observed for a transformation to massive martensite. 1n FIGURE 1 the structure as examined by electron transmission unpinned dislocations existing in a matrix relatively free of particles imparts both strength and toughness to the alloy. The macroscopic yield strength is strongly dependout upon the high strain hardening rate associated With such dislocation arrays. High toughness results from the ability of the dislocations to accommodate the imposed high strain rates of the Charpy test by rapid relaxation of stress concentrations. The relaxation rate depends in turn on the allowable plastic strain rate, 6, which is a fraction of the density of dislocation, n, and their velocity v, as given by:

where b is the Burgers vector. A high initial value for n leads to a spreading out rather than a concentration of .shear deformation, and reduces the average velocity of motion required. This latter effect is a particularly important consideration in iron based alloys at low temperatures where the velocity of motion of a dislocation as a function of stress decreases sharply. The influence of cooling rate on the mechanical properties is shown in Table 3 for 3Ni-3Mo steel. A rapid quench did not affect the tensile properties, but some improvement in toughness compared to air cooling was noted. If the alloy is very slowly cooled after rolling, the amount of equiaxed ferrite will of course increase. The grain size is much larger than the needles of massive martensite. The appearance of the equiaxed structure is shown in FIGURE 2 for a 3Ni alloy. Examination of thin foils reveals a marked decrease in dislocation content as compared to massive martensite. As a consequence both the impact and tensile properties are lessened. Similarly, if the alloys are either reheated after rolling and held at a temperature high in the ferrite range (1200-1400 F.) or furnace cooled from the rolling temperature, there is a loss in strength. The properties as a function of tempering temperature are listed in Table 4.

TABLE 4.EFFECT OF TEMPERING TEMPERATURE ON IMPACT TOUGHNESS [Alloy (VZ1006).0053C-3.05Ni-2.85Mo Plate A rolled at 1250 F. and Plate B rolled with cooling from 1850 to 1700 E] Temperature, Time, Hardness Charpy Condition F. hr. R.A. -l84 E),

ft.16 lb.

A (i1 240 600 l 6] 209 1, 000 1 (i2 240 1, 200 4 60 65 1, 400 1 50 240 1, 700 2 01 161 1, 900 1 60 9 02 69 l, 600 1 60 240 1, 700 1 s1 86 1, 800 1 61 35 When the material is reaustenitized there is no loss in properties provided that the temperatures and times are such as to avoid grain coarsening. For example, the properties after heat treatment at 1700 F. are much superior to those obtained at 1900 F.

The effect of varying the carbon content on the mechanical properties of the base alloys, 3Ni-3Mo-.7Mn-3Si- .05Nb, and 2.5Mn-3Cr-.3Si-.0.5N b is shown in Table 5.

TABLE 5.-EFFECT OF CARBON CONTENT ON PROPERTIES (A) Base Alloy 3Ni-3Mo Rolled at 1.400" F.

*.1V addition.

These results shown an increase in yield and tensile strengths with an increase in carbon content with air melted alloys generally stronger than those vacuum melted. The results of the impact tests are less straightforward. In vacuum melted 3Ni-3Mo alloys the impact strength remained high up to the 0.07 carbon level, but in the vacuum melted 2.5Mn-3Cr series there was a drop in impact resistance when the carbon level was increased even from .01 to .05. It is noteworthy that the addition of 0.1Al and 0.1V to air melt AZ2477 has increased the impact resistance at 40 F. to the 100 ft.-lb. range.

NICKEL AND MANGANESE The results of a series of tests on 3Mo-.7Mn-.3Si-.05Nb low carbon alloy heats in which the nickel content was varied are shown in Table 6.

TABLE 6.EFFECT OF NICKEL CONTENT ON PROPERTIES [Base alloy-.020-3Mo rolled at 1400 F.]

It is seen that an increase of nickel content for the same carbon level leads to an increase in the strength properties. Except for heat AZ2242, all of these alloys possess an impact strength at -40 F. in excess of ft.-lbs.

Results of a similar series of the 3Cr-Mn alloys in which the manganese content has been systematically varied have not been obtained. However, it is expected that the influence of manganese should be similar to that of nickel.

An important influence of molybdenum is to increase hardenability, and since this can also be accomplished by an increase in the manganese content, several heats were made of a 3Ni alloy in which both the molybdenum and manganese contents were varied. The results are given in Table 7.

TABLE 7.EFFECT OF MOLYBDENUM AND MANGANESE CONTENT ON PROPERTIES [Base alloy.02C-3Ni rolled at 1400 F.]

Number w/o w/o (2%), Ks.i. Percent Percent (40 F.),

Ks.i. it.-lb.

These results indicate that manganese alone (VZ1981) can lead to high toughness, but that in order for the yield strengths to reach to 100K s.i. level, the presence of a carbide forming element (=e.g., molybdenum) in this base alloy is needed.

MOLYBDENUM AND CHROMIUM These two elements have a similar effect on properties. Both increase hardenability, but chromium is also elfective in lowering the M temperature and both form alloy carbides in the same temperature range. These carbides are believed to form within the ferrite during the air cool after rolling.

The effect of molybdenum content on the properties is shown in Table 8.

*% inch rod heated at 1,650 F. for .5 hour and air cooled.

Over this range of molybdenum contents there is a monotonic increase in yield strength with increase in molybdenum content. The impact properties are a maximum in the 12% Mo range in this table; however, on occasion in alloys of higher molybdenum content with the same nominal compositions as VZ2052, e.g., VZ1996, impact values of 240 ft.-lbs. at 40 F. have been obtained.

The effects of chromium on the properties can be seen in the following Table 9.

TABLE 9.EFFECT OF CHROMIUM CONTENT ON TITANIUMALU MINUM-VANADIUM- NIOBIUM The effects of small additions of elements such as Ti, Al, V and Nb are difficult to specifically isolate because concerned. Any loss in hardenability due to the refinement of austenitic grain size does not appear to be significant in these alloys.

In the vacuum melts there is no beneficial efi'ect imparted by the addition of elements Ti, V and Al. In fact these additions can reduce the yield strength (e.g., VZ2185). This reduction may be due to the complete removal from solution of carbon by vanadium. In air melts, the addition of vanadium to alloys low in phosphorus and sulfur, has a beneficial effect on yield strength and the impact properties at 40 F. remain high (AZ2188). In fact the properties of all of the air melts are good, with the exception of AZ2192 which showed a reduction in impact strength for a high phosphorus and vanadium content. The addition of Ti and Al to air melts appears to be beneficial. Results of a similar study with Cr-Mn alloys are shown in Table 11.

TABLE 11.EFFEC'1 F VANADIUM, ALUMINUM AND NIOBIUM CONTENT ON PROPERTIES [Base alloy-O C-3Cr, 2.5Mn rolled at 1,400 and 1,750 E] Heat Rolling Y.S. (2%), Charpy Number V, w/o A1, w/o Nb, w/o Tempeirature, K 5.1. 1.S., K s.i. e, Percent R.A., Percent (1403s.),

17 045 05 1, 400 106 124 11 64 115 1, 750 105 124 11 74 95 AZ2553 19 040 055 1, 400 101 117 12 70 94 1, 750 99 115 11 74 116 AZ2554 16 057 1, 400 112 131 67 20 1, 750 110 130 10 71 95 AZ2555 03 060 1, 400 105 123 11 48 1, 750 100 117 11 100 AZ2556 17 1, 400 118 142 10 64 16 1, 750 113 135 10 72 11 035 1, 400 107 128 11 73 41 1, 750 102 121 11 75 15 of the many complex interactions possible. These elements have been generally added in quantities consistent with usual alloy steel practice. Throughout this investigation Nb has been a standard alloying element in the amount of 0.05 w/o. This usage of Nb is based upon preliminary work with rod material which revealed both an increase in the yield strength as well as the impact strength of alloys containing .02 w/o C. This effect of Nb can also be seen in the data for alloys VZ2011 and VZ2180, Table 10.

The properties of the first two heats indicate that the attainment of high impact strengths in Cr-Mn alloys is reproducible. These properties compare favorably with those of 3Mo-3Ni air melts. The omission of either vanadium or aluminum does not adversely affect the properties provided that the rolling temperature is at least 1700 F. (Heats AZ2555 and AZ2554.) The higher rolling temperature may facilitate the removal from solution of nitrogen by these elements. The absence of Nb leads to TABLE 10.EFFECT OF TITANIUM, ALUMINUM, VANADIUM AND NIOBIUM CONTENT ON PROPERTIES [Base alloy-3Ni-3Mo rolled at 1,400 F.]

Y.S. (2%), 1.S., R.A., Oharpy Heat Number C, w/o T1, w/o Al, w/o V, w/o Nb, w/o K s.1. K s.i. e, percent percent (14015.),

*Nominal.

The addition of Nb raises the yield strength and does not 65 a decrease in impact resistance, particularly at the higher impair impact properties at 40 F. This effect of Nb is thought to be due to its role in refining the grain size of austenite. In these low carbon alloys not enough of the NbC phase is formed to adversely aifect impact strength as has been found in the case of higher carbon alloys. Refinement of the grain size of the austenite will result in a smaller needle length of the massive martensite, and will also tend to distribute impurity particles over a larger total grain boundary surface area. Both rolling temperature (Heats AZ2556 and AZ2557).

THE EFFECT OF PHOSPHORUS AND SULFUR of these effects will be beneficial insofar as toughness is 75 listed in Table 12.

TABLE 12.EFFECT F PHOSPHORUS AND SULFUR CONTENT ON PROPERTIES [Base alloy-.02C-3Ni-3Mo rolled at 1,400 F.]

Heat Y.S. T.S., e, R.A., Charpy Number P, w/o S, w/o Tl, w/o Al, w/o V, w/o Nb, w/o (2%), K s.i. Percent Percent (40 F),

KSJ. ft.-lb.

The effect of these additions is not systematic, but it IS the high reduction of area of the low carbon massive seen that these elements can reduce the CVN values. 15 martensitic steels is indicative of good formability. Titanium appears to have an eifect which offsets the detrimental influence of phosphorus and sulfur on tough EFFECT OF COLD WORK ss. A plate of 3Ni-3 Mo-.02C alloy was reduced 84% by HARDENABILITY rolling at room temperature. The yield strength of the alloy was 240K s.i., its tensile strength was 207K s.i. and

In order to obtain an indication of the hardenability the elongation in 2" was 3%, as determined on flat tensile of these alloys a 2 /2 inch diameter ingot (AZ2531B) strips. The material still appeared tough in the sense that was sectioned in the transverse direction after casting. A failure was not brittle; and occurred as a result of plashardness traverse across the diameter of the exposed face tic instability with considerable lateral contraction in the was made and the hardness values were found to vary immediate vicinity of the fracture.

in a random manner from 18 /2 to 19 /2 Rc. A similar traverse was made on a section through the ingot which INGOT SIZE AND TRANSVERSE PROPERTIES had been reaustenitized and air cooled from 1750 F. One 40 lb. heat of a 3Mo-3Ni-.02C alloy was vacuum and similar readings were obtained. The alloy is theremelted and rolled to plate thickness in order to make fore fully hardened in section sizes at least up to 2 /2 a comparison with the properties of the usual 20 lb. heats.

inch diameter in both the as-cast and the air cooled con- The width of the plate was 4 inches which permitted transditions. verse as well as longitudinal properties to be obtained.

WELDABILITY The results were as follows:

The purposes of some initial tests on weldabihty were Ys" Ts, Percent Percent to determine 1s sound welds, free of cracks and blow Ks.i. Ksi. 6 RA. holes could be obtained by standard welding techniques, L 1 88 9g 15 79 and to assess the behavior of the heat affected zone. The Lggg ti mi crse 95 106 14 81 welding was performed by members of the Manufactur- IMPACT PROPERTIES mg Development Staff. Test weldments were made using T tT t .02C-3N1-3Mo steel plates. Orientation of Notch es empem um The weld material was e1ther of the same compositions -40" -120 as the plate with which a 75% argon-25%Co atmosphere was used, or Airco No. 352, low hydrogen, cov- 236 186 ered electrodes. The results of tests listed in Table 13 5 igff ilffgfigffiggffi fi ig gggg 186 144 indicated that crack free welds can be obtained and that ft.-1b 153 127 the heat affected zone is still quite tough, absorbing 44 to 104 ft.-lbs. Studies with 5Ni steels indicate that the These results indicate about the usual sort of anisotropy Tfinlsgerse and parallel to rolling plane,

transformation temperature of the present steels is suffiassociated with rolled plate products. The lower than ciently high to avoid cracking of the weld metal interface usual value of 88K s.i. for the yield strength of the alloy on cooling. Furthermore, the low carbon level prevents is thought to be due more to variation in carbon content the formation of carbides which are known to have an than to a loss in tensile properties as a result of increasing embrittling eflfect. the size of the ingot from 20 to 40 lbs.

TABLE 13.EFFECT OF WELDING ON IMPACT TOUGHNESS [Base alloy.02C-3Ni-3Mo rolled at 1,400 F.]

Weldment NumberFlller Metal Charpy Notch Position Relative to Weld VZ2179-9 VZ2179-4 AZ2186-7 AZ2186-8 AZ2186-6 AZ2188-2 AZ2l88-5 base l 352 base 1 Base l 352 Base 1 352 1. Notch in weld adjacent parent metal... 2 5O 89 72 2 6 47 2 17 48 2. Same as 1 2 9 100 2 9 2 2 104 2 21 44 3. Notch in patent metal removed from weld 240 240 141 2 5 139 80 112 4. Notch in center of weld 2 12 6 16 2 8 14 12 8 1 Holes and cracks present in weldment. 2 Partial fracture along weld interface.

FORMABILITY AS CAST PROPERTIES In order to determine the formability of these alloys, In many potential applications of high strength steels a /2 inch plate of the 3Ni-3Mo alloy was bent about a it may be desired to obtain the strength and toughness in /2 inch diameter through an angle of 90". No cracking the as-cast condition. In order to determine as-cast propwas observed. A correlation between the formability of erties therefore, the following alloys, containing additions high strength alloys and the reduction of area in the of either Zr or Hf for deoxidation, were cast and mechanitens est has been established, and on this basis also cal properties determined, Table 14.

TABLE 14.EFFECT OF HAFNIUM AND ZIRCONIUM CONTENT ON AS-CAST MATERIAL [Base alloy-.05C-3Ni-3Mo] Heat Number Hi, w/o Zr,* w/o Y.S. T.S., K s.i. e, percent R.A., percent Charpy K at. F.), 1t.-1b.

*Nominal composition.

The strengths of these as-cast alloys are similar to the (3) Johnston, L stoloff, S, and Davies, G. Phil alloys as-rolled, but there is a sharp decline in impact Mag" 12, 1965, 305

and ductility values. Nevertheless the properties seem to (4) Mangandlo S J Dabkowski D S Porter L F be of sufiicient interest to warrant further investigation. The effect of a normalizing treatment on several other g i i Metals Engmeermg Quarterly cast alloys was also studied, Table 15. e

TABLE 15.EFFEOT OF NORMALIZING TREATMENT ON PROPERTIES OF AS CAST MATERIAL [Base alloy-.02C-3Ni-3Mo held hr. at temperature] INormalizing As Cast 1,800 F. F- 2,l00 F.

emperature Cl H d Re Cha y Ha d s, Re Cl a y Heat Number Hardness, Rc 40g%a)1% lb. Hardness, Re (@109 F1 8I4) pft fir HESS b. r 1 65 (M100 l Jrypftrlb' 16 4 17 5 17 6 %ii::::::::::: i? 2 14 6 11 5 18 5 AZ2190 s 9 21 8 22 s .AZ2192 e 22 5 23 7 21 9 These results reveal that normalizing above 1800" F. (5) Marschall, C. W., Hehemann, R. F., and Troiano, has no significant effect upon the properties. The effect of A. R., Trans. ASM, 55, 1962, p. 135. lower normalizing temperatures is being investigated. (2) l\M/Ic}l:%v1}y, i1 Iand 1liuh, R.dI.,l;mpubh il1eid results. c vi y, u, an o nston, rans. FATIGUE PROPERTIES AIME, 236 1966, p 108 Fatigue tests were performed on a flNi-3Mo alloy hav- (8) Owen, W. S., Wilson, E. A., Bell, T., in High Strength ing a yield strength and tensile strength of 112K s.i. and M t ial d b V, Zackay, Joh Wil and So 133K s.i., respectively. Both smooth and notched spcci- N Y k, 1965, 3, 167, mens were tested as rotating beams (R. R. Moore type (9) Rolfe, 5, T, H k, R, P, d G o J, L1,, M t l The unnotched fatigue strength obeys the usual rule 1n Engineering Quarterly, 5, No. 1, Feb. 1965, p. 33.

that the fatigue endurance at long lives is about /2 of A (10) K b L L, M l P o r 33, 1965, p, 73, the tensile strength. The notched specimen (theoretical (11) D i K G unpublished lt notch fact of exhibits about the same degree of 12 International Nickel Co. pamphlet, ZOM 445-4342,

reduction of fatigue strength as other steels of this 19 5 Strength level- (13) Kaufman, L., and Ringwood, A. E., Acta Met. 9,

Because of the uniform structure of this steel it 1s 1961, p. expected that the degree of scatter of fatigue data should (14) Morrison, w, 13,, 1, I o nd Steel Inst, 201, 1963, be less than usual at these strength levels. 5 p.

Preliminary fatigue crack growth studies in sheet speci- (15) Steel, 150, 2, Jam 10, 1966, p 67.

mens revealed a characteristic of these steels associated (16) Metals Progress, 88, 2 August196i with the high lmpact toughness. The cracks grew until the (17) come, H Dislocations and Plastic Flow in stress on the remaining section was equal to the tensile Crystals, Oxford, 1953 strength of unnotched material, so that the material, (18) Patch, J Phil Mag 1, 1956 136 statlcany was notch msensltwe- (19) Meakin, J. D., and Petch, N. 1., Symposium on Role CARBURIZATION of Substructure, 1963, ASD-TDR-63-324, p. 243. (20) Ku, R. C., McEvily, A. J., and Johnston, T. L., to be In certain instances it may be desired to increase the published surface hardness by carburization. The effect on the properties of a carburized alloy (Z1755) Being not unmindful of the following patents, 2,707,680, was therefore investigated. The carburization was done succop May 1,955; Hayes July 1962; by the packed method at 70 0 F for 2 hrs. The 3,155,600, Hardwick 6t 31., 111116 2, 19645 and 703,9ll sultant surface hardness was 55 R and that of the in- (Bntlsh), 1954 clan as our mventlon: terior was 20 R In one case the notch was present prior The 2 of Producmg a lq strucufral Steel to carburizing and this specimen absorbed 17 ft.-lbs. in havmg m the asjroned condltlon, masswe mar' the Charpy impact test In a Second Set of Specimens, the tens tic structure essentially free of equiaxed ferrrte a notch was machined after carburization so that the base mmlmum yleld Strength of 80K a mmlmum of the notch was in a noncarburized region. These speci- Pact energy ab value at 40 of foot mens absorbed 95155 ft.-lbs., indicating that the core of Pounds Compnsmg fabncatmg an alloy haymg a {naxlmum the specimen retains its toughness after this heat treatment. carbon content of 007% alloymg metal The following partial bibliography is presented to assist selected from the group consisting of nickel, molybdenum,

in the understandingof this invention and its technical chromium: manganese and Silicon in an backgrouni stantially 1n excess of 7%, the sum of combinations of REFERENCES the nickel, molybdenum, chromium and manganese not exceeding about 6%, the alloy containing significant (1) Rinebolt, I. A., and Harris, W. 1., In, Trans. ASM, amounts of metal selected from the group consisting of 44, 1952, 225. nickel, manganese and combinations of nickel and man- (2) McMahon, C. J., and Cohen, M., Acta Met, 13, 1965, ganese such that the transformation of austenite to mas- 591. sive martensite is retarded at temperatures above 1300 F., the alloy further containing significant amounts of metal selected from the group consisting of chromium, molybdenum and combinations of chromium and molybdenum such that the transformation of austenite to equiaxed ferrite is essentially absent at temperatures above the austenite to massive martensite transformation temperature, the chemistry of the object being adjusted to exhibit an austenite to massive martensite transformation temperature below 1300" F. and a rate of transformation of austenite to massive martensite such that essentially no equiaxed ferrite is produced in air cooling from the austenite to massive martensite transformation temperature to room temperature, hot working the alloy and finishing the hot working operation at a temperature sufliciently low to inhibit substantial austenite grain growth during working and cooling down to the austenite to massive martensite transformation temperature, said object exhibiting a microstructure of massive martenite containing a high density of dislocations and metallurgically indistinguishable from the structure in FIGURE 1.

2. The process recited in claim 1 in which the alloy contains about 0.05% niobium.

3. A weldable structural steel object having in the asrolled condition a massive martensitic structure essentially free of equiaxed ferrite, a minimum yield strength of 80K 5i. and a minimum impact energy absorption value at -40 F. of 40 foot pounds, a maximum carbon content of 0.07%, a content of alloying metal selected from the group consisting of nickel, molybdenum, chromium, manganese and silicon in an amount not substantially in excess of 7%, the sum of combinations of the nickel, molybdenum, chromium and manganese not exceeding about 6%, the alloy containing significant amounts of metal selected from the group consisting of nickel, manganese and combinations of nickel and manganese such that the transformation of austenite to massive martensite is retarded at temperatures above 1300 F., the alloy further containing significant amounts of metal selected from the group consisting of chromium, molybdenum and combinations of chromium and molybdenum such that the transformation of austensite to equiaxed ferrite is essentially absent at temperatures above the austenite to massive martensite transformation temperature, the chemistry of the object being adjusted to exhibit an austenite to massive martensite transformation temperature below 1300 F. and a rate of transformation of austenite to massive martensite such that essentially no equiaxed ferrite is produced in air cooling from the austenite to massive martensite transformation temperature to room temperature, and exhibiting a microstructure of massive martensite containing a high density of dislocations and being metallurgically indistinguishable from the structure shown in FIGURE 1.

4. The weldable structural steel object recited in claim 3 and which includes a niobium content of about 0.05

5. The process recited in claim 1 in which the chromium, manganese, nickel and molybdenum are present in approximately equal amounts.

6. The object recited in claim 3 in which the chromium, manganese, nickel and molybdenum and present in approximately the same amounts.

References Cited UNITED STATES PATENTS 3,132,025 5/1965 Hurley 14836 X FOREIGN PATENTS 703,911 2/ 1954 Great Britain.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

H. SAITO, W. W. STALLARD, Assistant Examiners.

Claims (1)

1. THE PROCESS OF PRODUCING A WELDABLE STRUCTURAL STEEL OBJECT HAVING IN THE AS-ROLLED CONDITION, A MASSIVE MARTENSITIC STRUCTURE ESSENTIALLY FREE OF EQUIAXED FERRITE, A MINIUMUM YIELD STRENGTH OF 80K S.I. AND A MINIMUM IMPACT ENERGY ABSORPTION VALUE AT -40*F. OF 40 FOOT POUNDS COMPRISING FABRICATING AN ALLOY HAVING A MAXIMUM CARBON CONTENT OF 0.07%, A CONTENT OF ALLOYING METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, MOLYBDENUM, CHROMIUM, MANGANESE AND SILICON IN AN AMOUNT NOT SUBSTANTIALLY IN EXCESS OF 7%, THE SUM OF COMBINATIONS OF THE NICKEL, MOLYBDENUM, CHROMIUM AND MANGANESE NOT EXCEEDING ABOUT 6%, THE ALLOY CONTAINING SIGNIFICANT AMOUNTS OF METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, MANGANESE AND COMBINATIONS OF NICKEL AND MANGANESE SUCH THAT THE TRANSFORMATION OF AUSTENITE TO MASSIVE MARTENSITE IS RETARDED AT TEMPERATURES ABOVE 1300* F., THE ALLOY FURTHER CONTAINING SIGNIFICANT AMOUNTS OF METAL SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, MOLYBDENUM AND COMBINATIONS OF CHROMIUM AND MOLYBDENUM SUCH THAT THE TRANSFORMATION OF AUSTENITE TO

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