Background Of The Invention Traditionally, low alloy steels like AISI 4130 (1% chromium, 0.20% molybdenum) and 21CrMolO (2.25% chromium, 0.35% molybdenum) are considered the workhorse grades for pipe mold steel. The chemistry of these two grades, listed as weight percentages, is provided in Table 1, below. AISI 4130 chemistry is provided from Aerospace Structural Metals Handbook, (1986), pp 1-20. 21CrMolO chemistry is provided from Stahlschlussel (Key to Steel) (1977), pp 192-207. These low alloy steels have been used quite successfully over the years. However, the restrictions on the minimum amounts of certain elements have limited the specific minimum hardenability, as well as the mechanical properties of tensile and impact that can be achieved from either of these grades.

Table 1 Element AISI 4130 21CrMolO Carbon 0.28-0. 33 0.16-0. 23 Manganese 0.40-0. 60 0.20-0. 40 Silicon 0.20-0. 35 0.20-0. 40 Phosphorus 0.025 Maximum 0.025 Maximum Sulfur 0.025 Maximum 0.025 Maximum Chromium 0.80-1. 10 2.30-2. 60 Molybdenum 0.15-0. 25 0.30-0. 40 Nickel 0.25 Maximum------------- Copper 0.35 maximum------------- Iron Balance Balance Conventional thmking has been that pipe mold service life is primarily dependent on the properties of hardness and strength of the as-heat treated pipe mold. The main element in ferritic alloy steels that imparts hardness and strength to pipe mold steels is carbon. Therefore, it has been thought that to create pipe molds with long service lives there had to be high levels of carbon in the steel. Consistent with this thinking, the AISI 4130 grade had high carbon levels in the range of 0.28-0. 33%.

Modifications to the AISI 4130 grade chemistry have been previously made in assignee's U. S. Patent No. 4,992, 239. There also have been improvements to the 21CrMolO grade chemistry by the addition of vanadium.

There remains the need to produce pipe molds with properties that enhance the hardenability, while maintaining the strength, ductility and toughness under many types of working conditions. Additionally, this high hardenability and high strength can not be achieved at the expense of weldability in this steel.

Summary Of The Invention The present invention relates to a ferritic alloy steel with high hardenability, high toughness, and high ductility for making pipe molds used for centrifugally casting pipe. The steel of the present invention may preferably be used to make pipe molds or other products undergoing high thermal stresses.

Another object of the invention is to provide a steel for producing pipe molds with improved service life for centrifugally casting pipe. A further object of the invention is to provide a steel for producing pipe molds with improved service life for centrifugally casting pipe, with the pipe mold steel having a reduced carbon level and vanadium, manganese and silicon. Another object of the invention is to produce a steel of substantially high hardenability with the addition of manganese and silicon to the alloy system.

These and other objects of the invention will be described in detail in the remainder of the specification.

Detailed Description Of The Invention The present invention is directed to a low alloy ferritic steel for making pipe molds with improved service life that are used for centrifugally casting pipe. Pipe molds made from this steel can be used to centrifugally cast both small and large diameter pipe. The steel preferably may be used to make pipe molds or other products undergoing high thermal stresses. The present invention relates to a steel with high hardenability, high toughness, and high ductility. This invention creates an alloy that is a modification of the 21CrMolO base alloy system.

The alloying elements manganese and silicon, together with phosphorus, sulfur, chromium, molybdenum, nickel, and vanadium, provide desirable properties for long service life of pipe molds made with this steel. The combined effect of the use of manganese and silicon within the specified ranges, coupled with the other elements, promotes the enhancement of properties in the low chromium steel. The weight percentages of the steel of the present invention, which has been designated"0301" (Khare IV), are set forth in Table 2, below: Table 2 Element 0301 (Khare IV) Carbon 0.16-0. 22 Manganese 0.60-0. 90 Silicon 0.50-0. 65 Phosphorus 0.015 Maximum Sulfur 0.015 Maximum Chromium 2.25-2. 55 Molybdenum 0.40-0. 50 Nickel 0.55-0. 65 Vanadium 0.03-0. 08 Iron Balance The typical melt chemistry aims in weight percentages of the various elements is set forth in Table 3 below: Table 3 Element 0301 (Khare IV Carbon 0.18 Manganese 0.75 Silicon 0.56 Phosphorus as low as possible, 0.005 Maximum Sulfur as low as possible, 0.005 Maximum Chromium 2.40 Molybdenum 0.45 Nickel 0.50 Vanadium 0.05 Iron Balance Moreover, the alloying of the steel with manganese and silicon in the ranges specified promotes the desired toughness, hardenability, and ductility in the as-heat treated pipe molds. Pipe molds of a size ranging from 4 in. to 36 in. were prepared, heat treated, and tested for mechanical properties using the 0301 steel. The mechanical properties of the 0301 steel are set forth in Table 4, below.

Table 4 Size Tensile. 2% Yield % El % RA C-V-N Impacts (psi) (psi) (Foot Pounds, avg. of 3) 4 In. 119,000 107,000 23 75 154 6 In. 122,000 108,000 23 71 152 8 In. 123,000 111,000 23 74 147 8 In. 123,000 110, 000 22 73 153 8 In. 121,000 108,000 23 72 150 12 In. 128,000 116,000 20 71 142 20 In. 133,000 120,000 23 73 130 20 In. 133,000 119,000 22 74 129 24 In. 127,000 114,000 21 69 109 30 In. 126,000 113,000 21 66 115 36 In. 124,000 111,000 23 69 138 The carbon level of the steel chemistry of the present invention is lower than in the conventional AISI 4130 range of 0.28-0. 33%. Important here, the reduced carbon results in a reduction in hardness and strength coupled with an increase in toughness and ductility in the as-heat treated pipe mold. The reduced carbon also helps reduce the internal stresses of the steel of the present invention. This will mean that there is greater stability after tempering in the pipe molds made from the steel of the present invention. As such, the pipe molds will be less susceptible to quench cracking during the manufacture or due to thermal fatigue, and distortion during production. These combined effects greatly improve the service life.

Vanadium in the range of 0.03-0. 08% is added to the steel of the present invention to give the steel fine austenitic grain size and prevent softening during temper. Vanadium was not included in the AISI 4130 grade of steel. The fine grain size working in conjunction with the low stresses resulting from the use of reduced carbon enhances the stability of the steel of the present invention.

Vanadium, along with the alloying elements manganese and molybdenum, help maintain the desired level of post-temper hardness.

Manganese in the 0.60-0. 90% range provides a high carbon/manganese ratio. Manganese promotes deep hardening at the desired levels without adversely affecting the desired properties of toughness and ductility.

Additionally, manganese raises the strength without sacrificing weldability.

Silicon in the 0.50-0. 65% range provides Solid Solution strengthening and improves high temperature oxidation resistance. Silicon will counterbalance adverse temper embrittlement effects that may be caused by higher levels of manganese.

The terms and expressions that are used herein are terms of expression and not of limitation. And there is no intention in the use of such terms and expressions of excluding the equivalents of the features shown and described, or portions thereon, it being recognized that various modifications are possible in the scope of the present invention.