Plunger with ion nitriding treatment for a hydraulic fracturing pump and a method for making said plunger

Abstract

Steel plungers for hydraulic fracturing pumps having enhanced surface hardness properties, preferably made of alloyed steel and a method for manufacturing said plungers, comprising an ion nitriding process.

Claims

1. A plunger for a hydraulic fracturing pump, wherein the plunger is a steel plunger treated with an ion nitriding process, wherein the steel is selected from the group comprising AISI H13 steel, DIN 34CrAlNi 7 steel, SAE-AISI 4000 series steel or an equivalent alloyed steel having at least molybdenum.

2. The plunger according to claim 1, wherein the SAE-AISI 4000 series steel is a SAE-AISI 4140 steel.

3. The plunger according to claim 1, wherein the steel is a quenched and tempered steel.

4. The plunger according to claim 1, wherein the steel is quenched and subjected to a double-tempering treatment.

5. The plunger according to claim 1, wherein the plunger is further subjected to physical vapor deposition (PVD) surface treatment.

6. The plunger according to claim 5, wherein the PVD coating is about 5 m thick.

7. The plunger according to claim 6, wherein the PVD coating is a monolayer coating of elements selected from the group comprising Al, Cr and Ni.

8. The plunger according to claim 5, wherein the PVD coating is a monolayer coating of elements selected from the group comprising Al, Cr and Ni.

9. A method for manufacturing a plunger for a hydraulic fracturing pump, where the method comprises carrying out a surface treatment of a steel plunger by means of an ion nitriding process, wherein the steel is selected from the group comprising AISI H13 steel, DIN 34CrAlNi 7 steel, SAE-AISI 4000 series steel or an equivalent alloyed steel having at least molybdenum.

10. The method according to claim 9, wherein the SAE-AISI 4000 series steel is a SAE-AISI 4140 steel.

11. The method according to claim 9 wherein the steel plunger is further subjected to a double-tempering treatment.

12. The method according to claim 9 wherein following the ion nitriding process, the piston is subjected to a PVD surface treatment.

13. The method according to claim 12 wherein the PVD coating is about 5 m thick.

14. The method according to claim 13 wherein the PVD coating is a monolayer coating of elements selected from the group comprising Al, Cr and Ni.

15. The method according to claim 12 wherein the PVD coating is a monolayer coating of elements selected from the group comprising Al, Cr and Ni.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a prior art HVOF coating treatment of a metallic substrate

(2) FIG. 2 is a magnified microphotograph of a polished sample of a prior art plunger substrate with a HVOF coating, showing visible discontinuity between metallic substrate and the coating.

(3) FIG. 3 is a magnified microphotograph of a prior art polished sample of a plunger substrate coated with a HVOF coating, showing visible grain growth near the interface with the coating.

(4) FIG. 4 is a scanning electron microscopy (SEM) image of a prior art plunger substrate coated with a HVOF coating, showing visible adherence defects between the coating and the substrate.

(5) FIG. 5 shows a hardness profile for a substrate treated with ion nitriding according to the present invention.

(6) FIG. 6 is a schematic view of a typical installation for ion nitriding metallic articles used for carrying out the present invention.

(7) FIG. 7 shows a comparison between hardness profiles of various ion-nitride materials.

(8) FIG. 8 shows a tempering profile for AISI H13 alloyed steel used in the preferred embodiments of the present invention.

(9) FIG. 9 shows a comparison between hardness profiles of a prior art plunger and a plunger according to an embodiment of the present invention.

(10) FIG. 10 is a magnified microphotograph of a polished sample of a plunger according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(11) The hydraulic fracturing pump plunger with ion nitriding treatment of the present invention and the method for making the same will now be described in detail with reference to the accompanying drawings.

(12) As previously described, HVOF protective coating treatments used in prior art plungers comprise the deposition of external material by means of thermal deposition as illustrated in FIG. 1 (ASM Handbook, Vol. 5, Surface Engineering, ASM International, 1994). With this technique the resulting plunger has no metallurgic continuity between the substrate and the coating, that is to say, there is an interface between the outer coating and the metallic substrate, as can be appreciated in FIG. 2. This results in adherence deficiencies between the metallic substrate and the coating, as seen in FIG. 4.

(13) Additionally, the steps of heating and cooling of the substrate applied in one or more coating cycles cause an increase in the grain size of the metallic substrate at the interface, as can be appreciated in FIG. 3. This grain size increase results in the outer layers of the substrate having reduced toughness and hardness.

(14) With the aim of solving these problems, it is an object of the present invention to provide a steel plunger for a hydraulic fracturing pump comprising a treatment for enhancing its superficial hardness which does not produce discontinuity in the metallic structure of the metal substrate, nor produces grain size growth. In particular, it is an object of the present invention to provide a steel plunger for a hydraulic fracturing pump, provided with an ion nitriding treatment, which provides said plunger with enhanced surface hardness properties.

(15) Base Material:

(16) The accurate selection of the base material for the plunger of the present invention is of the outmost importance given that the response of the base material depends upon the presence of nitride-forming elements.

(17) As a way of example, when carbon steel is nitrided, the hardness of the layer is not higher than that of the core, since no further alloy elements are present which are capable to combine themselves with the available nitrogen in order to form nitrides for a potential hardening by precipitation. The chemical elements having a greater tendency to form nitrides, i.e., those which are desired for providing hardening by nitride formation, are Al, Cr, Mo, V and W.

(18) FIG. 7 shows comparative hardness curves for different steels (ASTM E 384-11, Standard Test Method for Knoop and Vickers Hardness of Materials, ASTM International, 2011). Among these materials, an alloyed steel having certain amount of alloy elements such as Cr, V and Mo, has been selected as the base material for an embodiment of the plunger of the present invention.

(19) The best base microstructure for these types of applications is tempered martensite, hence, the base metal shall be subjected to a thermal treatment of quenching and tempering. The tempering is a thermal treatment carried out for improving the metal toughness after quenching. In the case of steel for machines, the class of steel to which the selected material, i.e. alloyed steel, belongs, it is important to consider the possible incidence of secondary hardening or precipitation of alloy carbides at the high temperatures of the tempering process. It is possible to apply a double-, even triple-, tempering treatment for assuring an enhanced toughness after the microstructural changes induced by the first tempering process.

(20) FIG. 8 illustrates the hardness profiles as a function of the temperature of a preferred alloyed steel, AISI H13, subjected to tempering treatment (ASTM E 140-12b, Standard Hardness Conversion Tables for Metals Relationships Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, Sclerope Hardness and Leeb Hardness, ASTM International, 2012).

(21) Austenite in high-alloy steel for machines is very stable and does not completely transform until the temperature exceeds 500 C. On the other hand, in the particular case of alloyed steel tempered at temperatures between 475 C. and 535 C., it is possible that substantially flat and thick carbides be produced, which results in a reduced toughness to impact. To that end, a double-tempering treatment would tend to spheroidize interlaminar carbides formed by transformation of tempered austenite and reduce the detrimental effect thereof.

(22) Therefore, a double-tempered alloyed steel is preferred as the base material for the plunger of the present invention.

(23) Ion Nitriding:

(24) The ion nitriding technique is a surface hardening method that produces a change in the microstructure of a metal substrate, resulting in a localized hardening, but maintaining the continuity in the metallurgic structure between the substrate core and its surface.

(25) Ion nitriding of a substrate produces a layer of hardened material, the hardness curve of which is illustrated in FIG. 5 as a function of depth (ASM Handbook, Vol. 4, Heat Treating, ASM International, 1991). Said hardened layer comprises a diffusion zone and mayor may notinclude a compound zone. This depends upon type and content of alloy elements in the substrate, temperature and treatment time, as well as upon the mechanism used for the generation of nitrogen. The hardness of the compound zone is not affected by the alloy elements, while the hardness of the diffusion zone is governed by the nitride-forming elements: Al, Cr, Mo, Ti, V and Mn. X in the curve depends on the type and concentration of the alloy elements, while Y in the curve grows with temperature and decreases with the concentration of alloying elements. In view of these facts, it turns out that the accurate selection of the base steel is crucial, and that said steel should have a minimum, appropriately selected, alloying content.

(26) In a base steel, the original nitrogen (N.sub.2) is in solution within positions of the crystal lattice or as interstitial nitrogen, up to the solubility limit of Fe (0.4% by weight). If the N.sub.2 content is increased, coherent precipitates are formed, both at the grain boundaries and within the matrix i.e. interior of the grains. These precipitates, iron nitrides and other metals, distort the lattice, generating dislocations and thus, increasing the material hardness.

(27) This means that the nitrides formed by combining N.sub.2 with the alloying elements of the steel, because of being insoluble in the ferrite matrix, are precipitated right after their formation in a state of great dispersion, and the strain they cause to the crystal lattice is responsible for the hardening because it prevents the movement of dislocations. This phenomenon is known as precipitation hardening.

(28) In fine dispersions, ideally considered as randomly dispersed spheres within the matrix, there is a relationship between the achievable yield strength (Y) and the dispersion parameters (ASM Handbook, Vol. 4, Heat Treating, ASM International, 1991) as follows:

(29) $Y=0+\frac{T}{b.Math.A/2}$
where 0 is the yield strength of the matrix, T is the line tension of a dislocation, b is the Burgers vector thereof and A is the spacing between particles. This result arises from the analysis of the movement of dislocation around dispersed particles, showing that the lower the spacing in precipitated particles, the higher the hardening.

(30) The compound zone is a region wherein two types of intermetallic compounds are formed: (Fe.sub.4N) and (Fe.sub.2-3N). Carbon (C) promotes the formation of , whereby if the formation of this compound is promoted, methane gas (CH.sub.4) is incorporated to the ionizing chamber. Hydrogen (H.sub.2) tends to promote the formation of Fe.sub.2N. This layer of compounds, known as white layer due to its appearance upon metallography, is very hard and very fragile, and is characterized by a weak bond between phases, and different thermal phase expansion coefficients, a situation that should be taken into account when designing the treatment and performance thereof.

(31) In ion nitriding, carried out directly with N.sub.2 (as opposed to gas nitriding, where NH.sub.3 is used) it is possible to control whether or not the above described white layer exists. Given the layer characteristics of low toughness and the mechanical strength of the piece at service, it is sought in the plunger of the present invention to avoid the formation of the so-called white layer in the surface treatment to be implemented, thus, the use of ion nitriding is optimal in this sense.

(32) Ion nitriding (N.sub.2-based) shows some advantages over gas nitriding (NH.sub.3-based), such as: possibility of selecting a monophasic layer or or directly avoiding the formation of the white layer, higher control over the thickness of the hard layer, lower treatment temperatures, lower distortion of the piece, reduced environmental pollution (due to the fact that no NH.sub.3 is used), lower energy consumption, possibility of automation of the process, possibility of mask-coating those areas which are not to be nitrided.

(33) A facility for performing a nitriding process according to a preferred embodiment of the present invention is illustrated schematically in FIG. 6 (ASM Handbook, Vol. 4, Heat Treating, ASM International, 1991, pg. 421). Said process comprises the steps of: 1. placing the pieces to be nitrided, in this case the alloyed steel-plungers of an embodiment of the present invention, inside a nitrurating chamber (oven), on top of and in contact with a conductor plate connected to one of the terminals of an electrical source. The pieces are first subjected to an Argon ion sputtering surface cleaning process. Following said cleaning process an atmosphere comprising N.sub.2 and H.sub.2, and optionally CH.sub.4, is circulated into the chamber and a dynamic vacuum (about 5hPa) is established. 2. bringing said chamber to a pressure of about 0.1 mmHg; 3. heating the pieces until they reach a temperature of between about 375 C. and about 700 C. The heat is produced by electrical resistance heaters and by the ion bombardment itself; 4. establishing a potential difference between the pieces and the chamber (oven) enclosure of between about 500 V and about 1000 V (the enclosure of the chamber (oven) is at potential 0 due to grounding). This creates plasm that dissociates N.sub.2 forming N.sup. ions that impact on the pieces, which act as the cathode.

(34) The ion nitriding layer obtained by this process has an HRC Rockwell Hardness of between 50-70 and a thickness of between 75 m up to 0.75 mm, as required. The surface of the plunger, following the nitriding process, is mirror-polished.

(35) In a preferred embodiment of the present invention, following the ion nitriding process, the plunger may be subjected to an additional surface treatment such as PVD of atoms, ions, molecules. There are mainly three application techniques for PVD coatings: thermal evaporation, sputtering and ion-plating. Thermal evaporation implies heating of the material until a vapor is formed that condenses on the substrate and forms the coating. Sputtering implies the generation of a plasm between particles of the coating and the substrate, while ion-plating combines both the first and the second techniques.

(36) The PVD process for providing a coating to the plunger of the preferred embodiment of the present invention comprises the steps of: 1. synthesis of the material to be deposited (condensed state transition, solid or liquid to a vapor phase, or reaction between components of the compound to be deposited), 2. vapor transport from the source to the substrate, and 3. vapor condensation followed by a nucleation and growth process.

(37) The resulting PVD coating layer is a layer of about 3-5 m thick, which provides a hardness of about 3000 HV.

(38) Thus, in a preferred embodiment of the present invention, a plunger is provided that combines the aforementioned techniques, said plunger comprising: an alloyed base steel, such as AISI H13, DIN 34CrAlNi 7 or SAE-AISI 4140, a surface treatment by ion nitriding, a PVD deposition treatment.

(39) This selection of material and treatments results in a plunger with a substrate and structure as shown in FIG. 10, having high surface hardness, but without the defects derived from metallurgic discontinuity, grain growth and adherence deficiencies of the HVOF-treated plungers known in the art. A plunger manufactured by this process, according to a more preferred embodiment of the present invention, shows a surface hardness curve as a function of the depth as illustrated in FIG. 9, as compared to the hardness profile of the HVOF-treated plunger of the state of the art. The base metal selected, alloyed steel, shows an average hardness of 550 HV. Ion nitriding produces a diffusion zone that progressively increases hardness until reaching a value of about 1100 HV. Then, the PVD treatment creates a thin layer that further increases the surface hardness of the plunger until a hardness value near 3000 HV.

(40) Table 1 below depicts the plunger properties according to a preferred embodiment of the present invention, in terms of the base material, the ion nitriding treatment and the PVD treatment, as compared to an HVOF-treated plunger of the state of the art. As it may be appreciated therefrom, the enhanced hardness of the plunger of the invention is higher in all aspects in relation to the hardness of the plunger of the state of the art.

(41) TABLE-US-00001 TABLE 1 A plunger of the present Plunger of the state invention (AISI H13, ion of the art (HVOF) nitriding, PVD) Base Material 121 HV 550 HV Hard diffusion 1100 HV layer (layer thickness: 180 m) Hard surface 567 HV 3000 HV layer (1000 m) (layer thickness: near 5 m) Chemical feature Heterogeneous Homogeneous of the layer

(42) In conclusion, the plunger of the present invention shows the following advantages over the plungers used in the art: higher surface hardness, reduced surface roughness, transition between hard layer and core is of the gradual type, i.e. there is a metallurgic continuity, given by the hardness profile due to a diffusion structure, no adherence deficiencies of the substrate to the hard layer, and homogeneous layer, without zones having absence of hard material.

(43) Unless otherwise indicated, it should be understood that all numbers expressing quantities of components, process conditions, concentrations, properties, etc. used in the specification and claims are modified in all instances by the term approximately.

(44) The preferred embodiments and examples disclosed in the specification should not be construed as limiting the invention; they are included solely with explanatory and illustrative purposes for a better understanding of the invention, the scope of which is given by the appended claims.