Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-05-26
2001-03-13
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S320000, C148S660000, C420S087000, C420S089000, C420S008000
Reexamination Certificate
active
06200395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a free-machining steel which does not rely on lead as a means of enhancing machinability. More specifically, the invention relates to a free-machining steel having a concentration of tin, antimony, and/or arsenic at the ferrite grain boundaries of the steel which has machinability comparable to, or better than, that of conventional lead-bearing free-machining steels. The present invention also relates to a process for producing such free-machining steels.
2. Description of the Related Art
Free-machining steels are utilized in the machining of various components by means of fast-cutting machine-tools. Free-machining steels are characterized by good machinability, that is, (i) by their ability to cause relatively little wear on the cutting tool thereby extending the useful life of the cutting tool and (ii) by high surface quality. Low tool wear permits the use of higher cutting speeds resulting in increased productivity. The extended cutting tool life further reduces production costs by allowing savings in the cost of cutting tools and in the avoidance of the down time associated with changing cutting tools.
Machinability is a complex and not fully understood property. A full understanding of machinability would require taking into account a multitude of factors, including the effect of the steel composition, the elastic strain, plastic flow, and fracture mechanics of the metal workpiece, and the cutting dynamics that occur when steel is machined by cutting tools in such operations as turning, forming, milling, drilling, reaming, boring, shaving, and threading. Due to the complexities of the cutting process and the inherent difficulties in making real time observations at a microscopic level, knowledge of the extent of the range of mechanisms that affect machinability is also incomplete.
Metallurgists have long assumed that improvements in the machinability of free-machining steels could be obtained by modifying the chemical composition of those steels to optimize the size, shape, distribution, and chemical composition of inclusions to enhance brittleness of the chip and to increase lubrication at the tool/chip interface. They have also sought to prevent the formation of abrasive inclusions which could increase tool wear.
Accordingly, it has been common to use free-machining steels in which soft inclusions, such as manganese sulfide, are dispersed. The manganese sulfide inclusions extend cutting tool service life by bringing about effects such as crack propagation, decrease of cutting tool wear through tool face lubrication, and prevention of cutting edge buildup on the cutting tools. In contrast, hard oxide or carbonitride inclusions, such as silicon oxide, aluminum oxide, titanium oxide, titanium carbonitride, which have hardnesses higher than that of the cutting tool, act like fine abrasive particles to abrade and damage the cutting tool thereby decreasing its service life. Thus, free-machining steels are generally not subjected to strong deoxidation during steelmaking so as to keep the content of hard inclusions low.
Historically, lead has been added to free-machining steels containing manganese sulfide inclusions to enhance the machinability of those steels. However, the use of lead has serious drawbacks. Lead and lead oxides are hazardous. Caution must be taken during steelmaking and any other processing steps involving high temperatures. Such process steps produce lead and/or lead oxide fumes. Atmosphere control procedures must be incorporated into high temperature processing of lead-bearing steels. Disposal of the machining chips from lead-bearing free-machining steels is also problematic due to the lead content of the chips. Another serious disadvantage is that lead is not uniformly distributed throughout conventional steel products. This is because lead is not soluble in the steel and, due to its high density, it settles out during the teeming and solidification processes, resulting in segregation or non-uniform distribution within the steel.
Lead's ability to enhance machinability has been attributed to effects that flow from a combination of lead's low melting temperature and its propensity to surround manganese sulfide inclusions as a soft phase. Thus, previous efforts to replace lead in free-machining steels have focused on replicating this combination of characteristics. Consequently, face-machining steels were developed in which a soft phase, such as a low melting metal like bismuth or a plastic oxide, such as a complex oxide containing calcium, took the place of lead in surrounding the manganese sulfide inclusions.
SUMMARY OF THE INVENTION
The inventors have discovered a critical role that lead plays in enhancing the machinability of free-machining steels that is unrelated to lead's propensity to form a soft phase around sulfide inclusions. The inventors have discovered that lead causes an embrittling effect in free-machining steels at temperatures corresponding to the localized cutting zone temperatures which occur during machining. Through the use of hot compression tests, the inventors have discovered that, for lead-bearing free-machining steels, an embrittlement trough in the temperature range of about 200° C. to about 600° C. occurs in which the fracture mode changes from a relatively ductile transgranular mode to a relatively brittle intergranular mode.
FIG. 1
shows a graph of hot compression test results for two similar grades of conventional free-machining steels, one of which, AISI grade 12L14, contains lead, and the other, AISI grade 1215, does not. The deep trough in the graph for the lead-bearing 12L14 grade indicates an embrittlement region. Through microscopic examination of fracture surfaces, the inventors discovered that the embrittlement of the lead-bearing 12L14 grade was due to a change in fracture mode in the embrittlement temperature zone from transgranular to intergranular fracture.
The inventors further discovered that lead causes this embrittling change of fracture mode by being present at, and weakening, the ferrite grain boundaries of lead-bearing free-machining steel. Thus, the inventors discovered that lead resides at ferrite grain boundaries of the steel where, due to its effect on lowering the grain boundary cohesive strength, it causes the fracture mode to change from transgranular to intergranular in the temperature range corresponding to the localized temperatures occurring in the cutting zone during machining. Brittle, intergranular fracture requires relatively little energy input compared to ductile, transgranular fracture. Accordingly, the inventors further discovered that lead, by acting to embrittle the steel at the localized machining temperatures, improved machinability by reducing the energy input from the cutting tool necessary for cutting the steel, thereby resulting in less cutting tool wear.
Importantly, because of their discovery of this mechanism by which lead operates to improve the machinability of free-machining steels, the inventors were able to discover and solve a problem that was previously unrecognized by those skilled in the art. The inventors discovered that a problem to be solved in finding a substitute for lead in free-machining steels was to determine what could replace lead as an agent that resides at the ferrite grain boundaries to cause the fracture mode to change from transgranular to intergranular in the temperature range corresponding to the localized temperatures occurring in the cutting zone during machining. This discovery enabled the inventors to invent the free-machining steels of the present invention upon making their subsequent discovery that tin could act as such an agent and thus replace lead as a machinability enhancer in free-machining steels. Thus, the inventors made the surprising discovery that tin could replicate a machinability-enhancing effect of lead in free-machining steels.
Furthermore, the inventors have discovered that the machinability-enhancing effectiveness of a relati
DeArdo Anthony J.
Garcia C. Isaac
Lizzi Thomas
Metz Schermer & Lewis, LLC
Schermer Leland P.
University of Pittsburgh - of the Commonwealth System of Higher
Yee Deborah
LandOfFree
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