CA1103065A - Well casing steel - Google Patents

Abstract

ABSTRACT
a low alloy, high strength steel well casing exhibiting excellent resistence to hydrogen sulfide stress corrosion and high yield strength of from 90,000 to 145,000 psi comprised of an aluminum-killed steel alloyed with chromium, molybdenum and vanadium. A method of making such a steel including a quench and temper heat treatment characterized by the steps of austenitizing in the range of from 1550 to 1700°F, quenching to martensite, and tempering in the range of from 1200 to 1400°F
to a maximum hardness of 35RC.

Description

`` 11~ 3~5 The present invention relates generally to steel well casing, and more specifically to new and useful improve-ments in the manufacture of well casing characterized by superior resistance to hydrogen sulfide stress corrosion and high yield strength.
Considerable work has been done in recent years to develop higher strength casing steels which exhibit better resistance to failure under stress and corrosive conditions resulting from exposure to liquids containing hydrogen sulfide, as in sour oil applications. The need for higher strength hydrogen sulfide cracking resistant steels has become more apparent wi~h the increasing energy demands and the decrease of easily obtained sweet oil reserves. Oil fields now being explored require drilling to depths beyond 20,000 feet~with bottom hole pressures and temperatures exceeding 24,0qo psi and 400F, where hydrogen sulfide is often found in the crude oil. Under these conditions, steel well casing is progres-sively embrittled in the presence of the hydrogen sulfide and subsequently cracks and fails under the stresses to which the casing is subjected.
~5any metallurgical factors influence the sulfide stress cracking behavior of steel. These factors include the microstructure and composition of the steel and its strength level. All of these factors are interrelated and must be close-ly controlled. Small deviations from optimum limits of only one factor, such as the temperatures of heat treatment, will adverse-ly affect sulfide cracking resistance even though other factors such as composition remain unchanged.
Prior to the present invention, it had been generally concluded that casing steels having high yield strength levels of about 90,000 psi or higher were generally more susceptible to hydrogen sulfide stress cracking that lower strength steels.

The microstructures developed in quenched and tempered marten-r :' :

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sitic steels have been shown to be more resistant to sulfide stress cracking than those representative of the as-pierced or normalized condition or even the microstructures developed by normalizing and tempering. Chemical composition affects the resistance to hydrogen sulfide cracking of steels by changing such metallurgical characteristics of the steel as hardenability, transformation characteristics and tempering response which,~
in turn, result in changes in strength and microstructure~
The in~;uence of particular alloying and impurity elements on sulfide stress cracking resistance changes from one alloy system to another and the effects of these elements also dramatically change with changes in strength level. As a consequence, the effect on hydrogen sulfide stress cracking resistance of an element in one alloying system cannot be compared to o~r pre-dicated from the effect of that element in another system.
Although investigators have recognized the need for higher strength casing steels, the prior art has not provided a steel composition and a compatible heat ~treatment procedure that made it possible to increase the yield strength above 90,000 psi and at the same time improve the resistance to hydrogen sulfide stress cracking.
One prior art suggestion for improving the hydrogen sulfide stress corrosion resistance of casing steel is disclosed in U.S. Patent No. 2,895,861. It is proposed in that patent to use low allo~ steels containing chromium, molybdenum, vana-dium, silicon and manganese and to subject these steels to a heat treatment consisting of austenitization at an elevated temperature in the range of 1787 to 2012F. cooling at a speed - at least equal to air cooling, and tempering at a temperature in the range of from 1337 to 1472F. As distinguished from the present invention, Patent No. 2,895,861 specifies that the yield point of the steel should not be greater than 65kg./mm.2 or about 92,5000 psi. The patent teaches that tempering temperature ;3~65 below 1337F (725C) and yield strengths higher than 65kg./mm 2 (92,500 psi) are to be avoided in the case of the specifically dlsclosed steels which are austenitized and cooled in the manner described because of the detrimental effect on resistan~e to sulfide stress cracking.
Another hydrogen sulfide stress corxosion resistant steel is disclosed in U. S. Patent No. 2 r 825,669. The low , .

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.allo~ steel of tha~ patent contains as essential ingredients, in addition to an extremely close tolerance of carbon, small amounts of manganese, chromium, aluminum and siiicon An aluminum content of ~xom .15 to 1.20% by weight is disclosed as being required to promote and accelerate migration ana dispersions of carbides into the ferrite grains, impart stress corrosion resistance, and to strengthen ferrite. The composition also may include. as nonessential or optional in-gredients molybdenum, vanadium and titanium The steel is . 10 given a preliminary anneal by soaking at about 1364-1436~F for the apparent purpose of diffusing or dispers;ng the carbide . aggregates throughout the ferrite grains before the caxbides are dissolved b~ a subsequent high temperature austenitizing treatment. According to the patent disclosure, the steel ma~
be used in the state.obtained after the dispersion or diffusion treatment, o~ the steel may be subjected to an optional austen-itization, quench and temper treatment. The steel i5 auste~i-tized at a high tem~erature in the range of from 1778-1976F
The quenching treatment to which the steel is subjected after 20. austenitization may result in a microstructure containing martensite and other transformation products such as bainite, ; etc. As in the.case of U. S. Patent 2,895,861, the examples o~ Patent No. 2,825,669 involve steels heat-treated to produce yield strengths less than about 90,000 psi. .
The purpose of this.invention is to provide a casing.
steel which is not subject to the objections of the prior art, .and.more particularly a casing steel which exibits high strength levels as well as improved resistance to sulfide stress cor-. xosion cracking at any given strength level and, conversely, higher strengths at any given level of sulfide stress crackingresistance It has been found that an unexpected improvement o~
sulfide stress cracking resistance together with high yield strength can be achieved in a low alloy, fine grain, aluminum-killed steel containing carefully controlled amounts of molyb-~ 31~6~;
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denum, vanadium and chromium as essential ingredients by aquench and temper heat treatment earried out at critical temperatures to obtain a specified maximum hardness and a - fully tempered martensitic microstructure. The specifie eombination or interrelation of eomposition, heat treatment procedure and mierostructure is novel and achieves a syner-gistie effeet that is an improvement over the easing steel .

-3a-3~65i practices and compositions of the prior art in terms of improved sulfide stress cracking resistance and high strength. In par-ticular, the invention makes it possible to produce a well casing characterized by yield strengths ranging from 90,000 to 145,000 psi and by unexpectedly improved sulfide stress cracking resistance at any given strength level.
The invention provides a process of making well casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths of from 110,000 to 145,000 psi comprising the steps of providing an aluminum-killed steel consisting essentially in amounts by weight of from 0.15 to 0.35% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50%
silicon, 1.0 to 5.0% cromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, 0 to 0.25% columbium~and the balance iron, rolling and forming the steel into tubular form, austenitizing at a temperature of from 1550 to 1700F., quenching to obtain a microstructure which is essentially martensite, and tempering at a temperature of from 1200 to 1325F. to a maximum hardness of 35Rc.
The invention further provides a quenched and tempered well casing characterized b~ improved hydrogen sulfide stress cracking resistance at high yield strengths ranging from 110,000 to 145,000 psi comprising an aluminum-killed steel consisting essentially in amounts by weight of from O.lS to 0.35% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50% silicon, 1.0 to 5.0% chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55%
vanadium, 0 to 0.25% columbium and the balance iron, the steel having a maximum hardness of 35Rc and a microstructure which is essentially tempered martensite.
~lthough the composition of the new steel is similar to those suggested in U. S. Patents Nos. 2,895,861 and 2,825,669, the steel of the present invention is austenitized at a lower temperature in the range from 1550 to 1700F as compared to 3~

temperatures ranging -Erom 1778 to 2012F in the case of the patents. One reason for the improved sulfide stress crac~ing resistance characterizing the steel of the invention is be-lieved to be because the lower austenitization temperature avoids dissolution of the molybdenum, chromium and vanadium carbides. The lower austenitization temperature also results in a finer grain size. As further distinguished from the above-discussed patents, it is critical to the practice of the present invention that the steel be quenched to obtain-a microstructure consisting essentially of martensite. Another distinction is that the practice of the present invention involves tempering at a lower temperature to a yield strength of at least 110,000 psi. All of these factors are critical to obtaining the improved properties characterizing the present invention.
The hydrogen sulfide stress corrosion resistance of the new casing steels of the invention were evaluated by ex-posing notched cantilever loaded specimens to a hydrogen sulfide solution having a pH of 3. The tests were run for a period of 300 hours, and a survival stress level (designate~ as sigma 50) was statistically determined. The survival stress level repre-sents a median stress level above which 50% of the specimens failed in 300 hours and below which 50~ of the specimens did not fail in 300 hours. Tested by this method, casing steels of the invention exhibit sigma 50 survival stress levels rang-ing upward from about 115,000 psi.
Additional advantages and a fuller understanding of the invention will be had from the following detailed descrip-tion of specific examples of the invention.
Test specimens were made from two heats of steel having the compositions set forth 7 n the following Table I.
All of the specimens were austenitized at a temperature of 3~

1700F, quenched and tempered at temperatures ranging from 1150 to 1300F. The specimens were tempered to hardnesses ranging from 24.5 to 40.5 Rockwell C and yield strengths rang-ing from 125.6 to 165 ksi. The tempering temperatures, mechani-cal properties and the sigma 50 survival stress levels are pre-sented in Table II.

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Referring specifically to Table II, examples 3, 4, 7 and 8 were heat treated in accordance with the present inven-tion to a hardness less than 35Rc and a yield strength of from 110 to 145 ksi. Examples 1 and 2 were tempered to higher hardness of 40.5RC and 37.5Rc, respectively, and to higher yield strengths averaging 165 ksi and 151 ksi, respectively.
The survival stress level for example 1 was only 24.3 ksi and for example 2 only 48.3 ksi as compared to a survival stress level of 158.9 ksi for example 3 and 195 ksi for example 4.
Examples 5 and 6 were also tempered to high hardnesses of 40RC
and 37Rc, respectively, and yield strengths of 162 ksi and 148.7 ksi, respectively. The survival stress level was 24.3 ksi for example 5 and 51.3 ksi for example 6 as compared to 156.8 ksi for example 7 and 200.2 ksi for example 8.
Well casings were made from another heat of steel having the composition set forth in Table III. The casing steel was austenitized at a temperature of 1700F and tempered at a temperature of 1275F. Various casing sizes, and the mechanical properties and survival stress level of each size are presented in Table IV.

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Examples 9~ 10 and 11 of Table I~ have hardnesses and yield strengths as contemplated by the present invention.
It will be seen that the survival stress level ranges from 117 ksi for example 11 to 166.2 ksi for example 9. Example 12 has a higher yield strength of 147 ksi and a higher hardness of 35.3Rc. The survival stress level of example 12 was only 77.1 ksi as compared to 117 ksi for example 11. Example 13 had a yield strength within the scope of the present invention but a higher hardness of 35.1RC. The survival stress level of ex-ample 13 was 79.4 as compared to 117 ksi for example 11.
It will be seen from the foregoing that the invention makes it possible to obtain high strength casing steels having improved hydrogen sulfide corrosion stress resistance, and that these improved properties result from closely interrelated metallurgical factors of composition, hardness, strength, and microstructure.
Many variations and modifications of the invention will be apparent to those skilled in the art in light of the detailed disclosurP. Therefore, it is to be understood that, within the scope of the appended claims, the inven~ion can be practiced otherwise than as specifically described.

S _ LEMENTARY DISCLOSURE_ One aspect of the invention provides a process of makingwell casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths ranging from about 90 to 145 ksi comprising the steps of providing an aluminum-killed steel consisting essentially in amounts by weight of from 0.10 to 0.40% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50%
silicon, 1.0 to 5.0% chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, up to 1.0% aluminum, 0 to 1.0% nickel, 0 to 0.25%
titanium, 0 to 1.0% copper, 0 to 3.0% cobalt, 0 to 0.25% tungsten, 0 to 0.50% tantalum, 0 to 0.25% columbium, and the balance iron;
austenitizing the casing at a temperature of from 1500 to 1700F., quenching the casing to obtain a microstructure which is essentially martensite, and tempering the casing at a temperature of from 1200 to 1400F to a maximum hardness of 35Rc.
Another aspect of the invention is a new, quenched and tempered well casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths ranging from about 90 to 145 ksi comprised of an aluminum-killed steel consisting ess~ntially in amounts by weight of from 0.10 to 0.40% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50% silicon, 1.0 to 5.0% chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium and the balance iron, the steel having a maximum hardness of 35Rc and a microstructure which is essentially tempered martensite.
The steel composition characterizing the new well casing and the process of the invention is alloyed with 1.0 to 5.0% and more preferably 1.5 to 3.0% chromium, 0.30 to 1.0%
and more preferably 0.40 to 0.80% molybdenum, and 0.05 to 0.55% . .
and more preferably 0.10 to 0.30% vanadium.
Chromium has the effects of decreasing the diffusivity of hydrogen in iron and increasing the sulfide stress on~ - 12 3~

corrosion resistance of martensitic high-strength steels.
In addition, chromium is an alloy carbide former and provides a means for maintaining high-strength levels while increasing .

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-12a-3~G5 empering temperature. Increasing the chromium level from 1.0 to 2.0% in an alloy steel containing about 0.30% carbon, 0.72% molybdenum and 0.22% vanadium produced a pronounced beneficial effect on sulfide stress corrosion resistance. At the 1% chromium level, the sigma 50 survival stress level was about 146,000 psi, and at the 2% chromium level the sulfide stress corrosion resistance was raised to 196,000 psi.
Molybdenum also improves sulfide stress cracking resistance at yield strengths above 90,000 psi. In one test, the sigma 50 survival strength level of a medium carbon alloy steel ~0.30~ carbon, 2.0% chromium and 0.20% vanadium) increased from 163,000 psi to 191,000 psi with an increase in molybdenum content from 0.24 to 0.62%. A further increase in molybdenum content produced a decrease in hydrogen sulfide stress cracking thus indicating the optimum level to be in the range of from 0.60 to 0.70%.
Vanadium has been shown to produce a marked improvement in sulfide stress corrosion resistance in amounts up to about 0.20%. At higher vanadium levels, for example levels in excess of about 0.30%, no additional improvement in sulfide stress cracking resistance was observed. The beneficial effect of vanadium is thought to result from its role as a carbide former which allows the use of higher tempering temperatures to reach a given strength level.
The steels characterizing the product and process of this invention are aluminum-killed and, as such, contain aluminum in amounts of from about 0.02 to 0.10% and more preferably about 0.02 to 0.08%. At these low levels resulting from the use of aluminum as a deoxidizer, there is no marked effect on sulfide stress cracking resistance. Tests conducted with chromium-molybdenum-vanadium alloyed steels have shown that amounts of aluminum greater than about 0.10%, and more particularly in the range of from 0.30 to 0.41%, result in significantly -\` ~i poorer sulfide stress cracking resistance at all strength 6~
evels as compared to the steels of the present invention.
Columbium may be illcluded in the composition as an optional ingredient in amounts up to about 0.10%. Tests have shown that colun~ium causes a slight i~provement in sulfide stress cracking resistance up to an amount of about 0.05%.
No appreciable benefits are observed by increasing the columbium content above this level.
Other optional ingredients are nickel up to about 1.0%, titanium up to about 0.25~, copper up to about 1.0~ cobalt up to about 3.0%, tungsten up to about 0.25%, and tantalum up to about 0.50%. If desired, these optional ingredients may be present in the steel in the maximum amounts indicated without producing any pronounced effect on sulfide stress cracking resistance.
' Test specimens were made from four heats of steel having the compositions,set forth in the following Ta~le V.
The first two heats of Table V are the same heats as in Table I~
All of the specimens were austenitized at a temperature of '' 1700F., quenched and tempered at temperatures ranging from 1150 to 1300F. The specimens were tempered to hardnesses ranging from 24.5 to about 40.5 ~ockwell C and yield strengths ranging from about 110 to 166 ksi. The tempering temperatures, mechanical properties and the sigma 50 survival stress levels are presented in Table VI, in which the first eight examples are the same ones as in Table II.

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The criticality of the heat treatment procedure, especiall~
the step of tempering at a temperature of at least 1200F., is demonstrated by Example 1 from heat No. X439 which was tempered at a temperature of about llsOF. Examples 2, 3 and 4 from the same heat were tempered at temperatures about 1200F., 1250F.
and 1300F., respectively. It will be seen that the sigma 50 survival stress level, which factor is a measure of the hydrogen sulfide stress corrosion resistance, was only 21.1 ksi in the case of Example l.. as compared to 48.3 ksi, 163.8 ksi and 195 ksi for Examples 2, 3 and 4, respectively.
' The unexpected.`! advantageous effect of the higher tempering .
temperature required by this invention is also demonstrated by a comparison of Example 5 to Examples 6,7 and 8, Example 14 to Examples 15, 16 and 17, and Example 18 to Examples 19, 20 and 21.
Example 5 tempered at 1150~. had a survival stress level of 21.1 ks~, whereas Examples 6, 7 and 8 from the same heat! tempered at temperatures of 1200F., 1250F.,and 1300F., had survival stress levels of 50.0 ksi, 161.6 ksi and 200.2 ksi, respectively. Example 14 tempered at 1150F. had a survival stress level of 31.1 ksi, whereas the same steel tempered at temperatures of 1200F. r 1250F.
,. and 1300F. had survival stress levels of 74.0 ksi, 207.4 ksi and 195.2 ksi, respectively. Example 18 tempered at 1150F.
had a survival stress level of 21.1 ksi. The survival stress level of the steel at 1300F. ~Example 21) was 177.9 ksi. The values for Examples 19 and 20 were 44.4 ksi and 126.0 ksi, respectively.
The criticality of heat treating to a hardness less than 35 RC and yield strengths up to a maximum of 145 ksi also is demonstrated by the examples reported in Table VI. Examples 1 and 2 were tempered to higher hardnesses of 40 7 RC and 37Rc, respectively, and to higher yield strengths of about 164.9 ksi ~16-~\ ~
~31)~i5 and 151.4 ksi, respectively. Examples 3 and 4 from the same heat were heat treated in accordance with the present invention and had survival stress levels of 163.8 ksi and 195.0 ksi, respectively Similar results were obtained with'the specimens made from the other three heats of steeI X440, X441 and X442. Examples'5, 6, 14, 15, 18 and 19 were all heat treated to hardnesses and yield -strengthS exceeding the'ranges of the present invention, i.e., 35 RC and 145 ksi. Examples 7, 8, 16, 17, 20 and 21 were heat treated to hardnesses less than 35 RC and yield strengths of from 90 to 145 ksi. The hydrogen sulfide corrosion resistance was substantially higher with'the'survival stress levels ranging from 126.0 ksi in the case of Example 15 to 207.4 ksi in the case of Example 16.
- The'effects of certain compositional variances also are made evident by the results presented in Table VI. In particular, ~xamples 18, 19, 20 and 21 made 'from heat X442 demonstrate the adverse effects of a columbium content exceeding about 0.10~. ' ' In the case of heat X442, the columbium content was about 0.12%
and the resulting sulfide stress corrision resistance at each reported strength level was significantl~ poorer than that of ' 20 steels made in accordance with'the'invention. Example 20 from heat X442 had a survival stress value of 126.0 ksi at a strength level of 131.3 ksi, l~hereas Examples 3 and 16 which' had lower columbium contents within the range of the invention had survival stress values of 163.8 ksi and 207.4 ksi at substantially'the same strength level. Example 21 from heat X442 had a survival stress value of 177.9 ksi at a strength level of 113.9 ksi, whereas Examples 4, 8 and 17 had higher survival stress values of 195.0 ksi, 200.2 ksi and 195.2 ksi, respectively, at nearly the same strength level.

Examples 22 and 23 demonstrate the adverse effect of in-creased aluminum content above that required simply for deoxid-3~65 ization, i.e., up to about 0.10~. The steels for these examples had aluminum contents of 0.30 and 0.39, respectively. The survival stress value for Example 23 was 149.7 at a strength level of 108.1 ksi, whereas Example 8 had a survival stress value of 200.2 ksi at the same strength level. Example 23 had a survival stress value less than about 120.0 ksi at a strength level of 126.8 ksi, whereas Example 7 had a survival stress level of 161.8 ksi at about the same strength leveI.

Well casings were made from another heat of aluminum-killed steel having the composition set forth in Table VII. The casing was austenitized at a temperature of 1700F. and tempered at temperatures ranging from 1200 to 1350F. The mechanical properties and survival stress levels are presented in Table VIII.

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,1 ' -19-3a~5 Example 24 was heat treated to a hardness and yield strength outside the ranges of the present inVentiQn. The survival stress level was 69.1 ksi. Example 25 was heat treated to a hardness and yield strength at the outer limits of the invention (about 35 RC and 145 ksi). The survival stress level of 106.3 ksi was appreciably higher than that of Example 24.
Examples 26, 27 and 28 were heat treated within the preferred ranges of the invention. The survival stress levels ranged from about 187 ksi for Examples 26 and 28,:to 199.5 ksi for Example 10, 27.
It has been demonstrated by all of the foregoing exa,mples that precise control of composition and heat treatment within the ranges specified by the invention are needed to obtain the best hydrogen sulfide corrosion stress resistance. Seemingly slight variations in chemistry, such as increasing the columbium ' or aluminum contents as in the ~ase of Examples 20-23, while .
maintaining all other parameteres of the invention, result in ~arkedly poorer sulfide stress corrosion resistance at any given strength'level. Conversely, slight variations in heat treat-'20 ment prodedure while maintaining the chemistry within the rangesof the invention decreases sulfide'stress corrosion resistance.
It will therefore be seen that the specific interelation of chemistry and heat treatment as taught by the invention produces improved, synergistic results.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS.

1. A process of making well casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths of from 110,000 to 145,000 psi com-prising the steps of providing an aluminum-killed steel consisting essentially in amounts by weight of from 0.15 to 0.35% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50%
silicon, 1.0 to 5.0% chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, 0 to 0.25% columbium, up to 1.0%
aluminum and the balance iron, rolling and forming the steel into tubular form, austenitizing at a temperature of from 1550 to 1700°F., quenching to obtain a microstructure which is essentially martensite, and tempering at a temperature of from 1200 to 1325°F to a maximum hardness of 35RC.

2. Well casing steel exhibiting improved hydrogen sulfide stress corrosion resistance comprised of an aluminum-killed steel consisting essentially in amounts by weight of from 0.15 to 0.35% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50% silicon, 1.0 to 5.0% chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, 0 to 0.25% columbium, up to 1.0%
aluminum, and the balance iron, said steel having a yield strength of from 110,000 to 145,000 psi, a microstructure consisting essentially of tempered martensite, and a maximum hardness of 35RC.
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE:

3. A process of making well casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths ranging from about 90 to 145 ksi comprising the steps of providing an aluminum-killed steel consisting essentially in amounts by weight of from 0.10 to 0.40% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50%
silicon, 1.0 to 5.0% chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, up to 1.0% aluminum, 0 to 1.0%
nickel, 0 to 0.25% titanium, 0 to 1.0% copper, 0 to 3.0%
cobalt, 0 to 0.25% tungsten, 0 to 0.50% tantalum, 0 to 0.25%
columbium, and the balance iron; austenitizing the casing at a temperature of from 1550 to 1700°F., quenching the casing to obtain a microstructure which is essentially martensite, and tempering the casing at a temperature of from 1200 to 1400°F to a maximum hardness of 35RC.

4. A process of making well casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths ranging from about 90 to 145 ksi comprising the steps of providing an aluminum-killed steel consisting essentially in amounts by weight of from 0.10 to 0.40% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50%
silicon, 1.0 to 5.0% chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, 0.02 to 0.10% aluminum, 0 to 0.10%
columbium and the balance iron except normal steel making impurities; rolling and forming the steel into tubular form;
austenitizing the casing at a temperature of from 1550 to 1700°F., quenching the casing to obtain a microstructure which is essentially martensite, and tempering the casing at a temperature of from 1200 to 1400°F. to a maximum hard-ness of 35RC.

5. A process of making well casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths ranging from about 90 to 145 ksi comprising the steps of providing an aluminum-killed steel consisting essentially in amounts by weight of from 0.15 to 0.35% carbon, 0.35 to 0.65% manganese, 0.10 to 0.35%
silicon, 1.5 to 3.0% chromium, 0.40 to 0.80% molybdenum, 0.10 to 0.30% vanadium, up to 0.08% aluminum, 0 to 0.10%
columbium, and the balance iron except normal steel making impurities; rolling and forming the steel into tubular form;
austenitizing the casing at a temperature of from 1550 to 1700°F., quenching the casing to obtain a microstructure which is essentially martensite, and tempering the casing at a temperature of from 1200 to 1400°F. to a maximum hard-ness of 35RC.

6. A process according to claim 3, 4 or 5, wherein the steel includes 0.01 to 0.05% by weight columbium.

7. Quenched and tempered well casing charac-terized by improved hydrogen sulfide stress cracking resis-tance at high yield strengths ranging from about 90 to 145 ksi comprised of an aluminum-killed steel consisting essen-tially in amounts by weight of from 0.10 to 0.40% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50% silicon, 1.0 to 5.0%
chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, up to 1.0% aluminum, 0 to 1.0% nickel, 0 to .25% titanium, 0 to 1.0% copper, 0 to 3.0% cobalt, 0 to 0.25% tungsten, 0 to 0.50% tantalum, 0 to 0.25% columbium, and the balance iron, the steel havig a maximum hardness of 35RC and a micro-structure which is essentially tempered martensite.

8. Quenched and tempered well casing charac-terized by improved hydrogen sulfide stress cracking resis-tance at high yield strengths ranging from about 90 to 145 ksi comprised of an aluminum-killed steel consisting essen-tially in amounts by weight of from 0.10 to 0.40% carbon, 0.25 to 0.75% manganese, 0.05 to 0.50% silicon, 1.0 to 5.0%
chromium, 0.30 to 1.0% molybdenum, 0.05 to 0.55% vanadium, 0.02 to 0.10% aluminum, 0 to 0.10% columbium, and the balance iron except normal steel making impurities, the steel having a maximum hardness of 35RC and a microstructure which is essentially tempered martensite.

9. Quenched and tempered well casing characterized by improved hydrogen sulfide stress cracking resistance at high yield strengths ranging from about 90 to 145 ksi comprised of an aluminum-killed steel consisting essentially in amounts by weight of from 0.15 to 0.35% carbon, 0.35 to 0.65% manganese, 0.10 to 0.35% silicon, 1.5 to 3.0% chro-mium, 0.40 to 0.80% molybdenum, 0.10 to 0.30% vanadium, up to 0.08% aluminum, and 0 to 0.10% columbium, and the balance iron except normal steel making impurities, the steel having a maximum hardness of 35RC and a microstructure which is essentially tempered martensite.

10. Well casing according to claim 7, 8 or 9 wherein the steel includes 0.01 to 0.05% columbium.