Environmentally assisted cracking or EAC refers to the attacks including stress corrosion cracking (SCC), hydrogen embrittlement (HE), sulfide stress cracking (SSC), liquid metal embrittlement (LME) and corrosion fatigue (CF). Under EAC, generally a ductile metal looses its hardness when mechanical load is applied in presence of a specific corroding media. The simultaneous presence of three factors: mechanical tensile stresses, sensitive metal microstructure and particular severe media result in the EAC of a metal. Prevention of any of these of factors controls EAC. Therefore to prevent EAC, engineers discard residual stress in a material or control its use to a specific severe chemical.

Properties of Nickel alloys

Nickel based alloys are solid solution based materials that contain a large percentage of Nickel as well as other alloying agents. Nickel alloys have face centered cubic lattice structure from the nickel base element. Resulting to FCC structure, Nickel based alloys offer outstanding ductility, malleability and formability. They are easily weldable. Commercial Nickel based alloys are categorized in two major commercial groups: one group is made to withstand high temperature and dry or gaseous corrosion, second group is made to low temperature (aqueous) applications. Nickel alloys are utilized for low temperature aqueous or condensed systems are generally called as corrosion resistant alloys. The nickel alloys utilized for high temperature applications are called as heat resistant alloys or high temperature alloys. The practical industrial limit between high and low temperature nickel alloys is in the range of 500oC or 1000oF. Several nickel alloys have an evident use in corrosion resistance or heat resistance based applications, although some alloys can be utilized for both applications.

Heat Resistant Alloys

Rather corrosion resistant alloys that are widely chosen for their capacity to resist corrosion in specified condition, various heat resistant alloys require playing a double role. In addition their capacity to endure the corrosive severity of the condition, heat resistant alloys also require to have a considerable strength at the elevated temperatures. In several conditions, for instance near and exceeding 1000oC, the choice of alloy is dominated by how the alloy strong is in this temperature limit. There are several diverse industrial elevated temperature conditions. Generally the practical use has categorized these conditions as per the several common causes of damage of a product in a service. Various damages are related to the corrosion by a particular agent like oxygen, carbon, sulfur, halogen and nitrogen. Other damaging models like molten metal corrosion and hot corrosion are less particular. The major high temperature degradation mode oxidation and security from oxidation is described by the production of a chromium oxide layer. In several conditions, the availability of nominal magnitude of aluminum or silicon in alloy may enhance the resistance from oxidation of a chromia-producing alloy. Corrosion by several elements like chlorine and sulfur depends significantly on the partial pressure of oxygen in the condition. The damages of heat alloys in application are not often related with conditions accelerated cracking because they may not be in link with a condensed phase that increased failure.

Chemistry and corrosion nature of corrosion resistant alloys

Nickel alloys are extremely resistant to corrosion in several conditions, they offer extensive performance over the advanced stainless steels. The major reasons nickel is alloyed more extensively than iron are that considerable amounts of specific elements can be dissolved knowingly into nickel to produce alloy for application in specific conditions. Generally the industrial media can be categorized into major classes- reducing and oxidizing. These refer to range of electrode potential that the alloy’s use is controlled by cathodic reaction in the system. Hence a reducing condition is normally monitored by the discharge of hydrogen from a reducing acid like hydrochloric acid.

From the point of chemical composition, Nickel based alloys are – 1. Pure Nickel  2. Nickel-Copper alloys  3. Nickel-Molybdenum alloys  4. Nickel-Chromium-Molybdenum alloys  5. Nickel-Chromium-Iron alloys. Nickel is nominally attacked in hot caustic solutions than alloyed nickel as the composition elements such as chromium and molybdenum dissolve significantly in the hot caustic solutions.

Nickel can also perform in cold reducing acids due to slow discharge of hydrogen on its surface. The warm reducing acids and oxidizing acids attack pure nickel quickly. The chief use of Nickel-Copper alloys is in dealing pure hydrofluoric acid. However if hydrofluoric acid is contaminated with oxygen, Ni-Cu alloys may be attacked by intergranular corrosion. These alloys offer nominally more corrosion resistance than pure nickel in pure reducing and oxidizing acids like sulfuric acid and nitric acid.

Nickel-Molybdenum alloys or Hastelloy B alloys are particularly made to perform in the reducing HCl conditions at the whole magnitude and temperature levels. In addition of comprising of costly element tantalum, Ni-Mo alloys are the best material for use in warm hydrochloric acid. These can successfully prevent corrosion in all types of reducing conditions such as sulfuric, acetic, formic, hydrofluoric and phosphoric acids. But these alloys cannot perform satisfactorily in conditions including oxidizing agents such as hydrochloric acid mixed with ferric ions. Here the need of Nickel-chromium-Molybdenum alloys such as Hastelloy C276 comes in which controls attack in both reducing and oxidizing media. Ni-Cr-Mo alloys have major applications in warm chloride based solutions. In these conditions, many stainless steels suffer from crevice and pitting attack and stress corrosion cracking.

The major drawback of stainless steels is their sensitivity to chloride induced localized corrosion such as crevice and pitting corrosion and SCC. Ni-Cr-Mo alloys are highly resistant materials in Nickel based alloys to the classic chloride based localized corrosion that limits the use of stainless steels. In few conditions, SCC occurs in high strength materials, although it only occurred in severe conditions at temperatures above 200oC, pH below 4 and in availability of hydrogen sulfide. Hastelloy C2000 is found not to be sensitive to cracking in boiling 45 % magnesium chloride solution even after 1008 hours. The resistance of Ni-Cr-Mo alloys to corrosion in wet HF is due to the advantageous effect of copper content by 1.6 %.

Nickel alloys have higher corrosion resistance than stainless steels to EAC. The stress corrosion cracking of austenitic stainless steels occurs in hot aqueous solutions consisting of chloride ions. Because chlorides are an integral part of most industrial chemical procedures, employing stainless steels is seriously not recommended due to the risk of chloride cracking attack. Nickel alloys such as Ni-Cr-Mo alloys are used to prevent SCC in such hot chloride solutions and hence these prove to become the best replacement to the austenitic stainless steels.

The Hastelloy C alloys family consists of nickel-chromium-molybdenum alloys that are used in the extensive range of applications because of their capability to successfully withstand the oxidizing and reducing media. Commonly the Hastelloy C alloys offer modest corrosion resistance properties in hot concentrated sulfuric acid as compare to other Nickel based alloys. Although every pump manufacturer is concerned in improving the functionality of C alloys in these applications as the chemical units are often using pumps in direct or indirect manners to handle the dilute concentrated sulfuric acid. Hastelloy alloys have significant applications in severe Flue gas desulfurization conditions.

Oxidation behavior

It is widely accepted that Ni-Cr-Mo alloys offer outstanding corrosion resistance in the severe conditions therefore they are used in preparation of elevated temperature flue gas filter with extensive gas corrosion, high temperature and wide diffusion ate of oxygen. With wide complexity and increasing temperature of service media, it is a major challenge for alloys to sustain their outstanding characteristics such as elevated temperature corrosion in oxidizing and reducing conditions and extensive oxidation resistance.

Newly invented Hastelloy C-2000 offers very promising performance in the high temperature flue gas filter as it resists attack in the intensely corrosive conditions. The outstanding corrosion resistance of alloy C2000 is because of large magnitude of Molybdenum element that can prevent proliferation and movement of corrosion. The corrosion resistance of alloy is affected when the molybdenum element is continually consumed during the elevated temperature oxidation. As the grain boundaries can increase the oxidation rate of alloys because of their high density defects. However density of defects by annealing is smaller than the grain edges.

Hastelloy C2000 Introduction

Hastelloy C2000 is a face centered cubic lattice with small stacking fault energy, annealing twins are readily produced subsequent to mill annealing. The lamellar twin created at the trigeminal grain boundary by collecting fault nucleation derived from the presence of stacking faults, results in decreasing the interfacial energy and mean free path of dislocation. The development of oxide scales is monitored by external diffusion of interstitial cation. The development rate of oxide layer is extensively larger in annealing twins as compare to that in other regions of large density of the interstitial cation.

Surface oxide layers produced on Hastelloy C-2000 samples are developed in different colors and bump structures after isothermal oxidation. The oxide layers on annealing as well as non-annealing twins are comparatively uniform derived from a balanced growth stress and spalling from alloy substrate is inhibited. The oxides of manganese and iron are produced on the sample’s surfaces at the temperatures above 1000oC. The content of molybdenum in oxide is more at 800oC than 1000oC. At 1000oC, the oxide layer’s thickness is about 8 micro-m and internal oxide hooks also develop into alloy matrix that enhances the interaction force between oxide scales and alloy matrix.

Hastelloy C2000 is the topmost superalloy offering unrivaled character in all kinds of possible acidic conditions (both reducing and oxidizing type)

Hastelloy C2000 offers excellent resistance to sulfuric acid, hydrochloric acid, hydrofluoric acid, phosphoric acid, organic acid, chlorides, alkalis, pitting and crevice corrosion and stress corrosion cracking. Heanjia Super-Metals produces complete line of product forms of Hastelloy C2000 settled by ASME and ASTM standards. Alloy C2000 is easily formed, machined and welded into the industrial parts.

Alloy C2000 is featured with broad applicability among the series of Hastelloy alloys. It is manufactured to prevent corrosion in the wide range of corrosive chemicals. As compare to traditional Nickel-Chromium-Aluminum (Ni-Cr-Mo) alloys that were made for applications in oxidizing or reducing media, alloy C2000 can be successfully used in both kinds of conditions. Its composition contains molybdenum and copper that offer great resistance in reducing conditions while resistance to oxidizing acid is given by a considerable magnitude of chromium about 23%.

Considering its role in engineering sector, C-2000 alloy has significant potential for manufacturing enhancement in the plant applications. It replaced the traditional Hastelloy alloys providing better corrosion resistance in the specific conditions as well as enhanced equipment’s service life such as reactors, valves, pumps and heat exchangers, in the troubling condition. Its contribution in the industrial procedures has significantly lowered the process cost. For instance, a reactor made for handling hydrochloric acid extract can later be used in handling the process that may involve use of nitric acid. The multi- uses of Hastelloy C2000 alloy make it an outstanding nickel based alloy in handling the various process streams.

Machining

Following the benefits of nickel based alloys, they are more widely used in the industry than titanium based alloys, aluminum and others. Nickel based superalloys are heat resistant materials due to which they sustain their chemical and mechanical characteristics at the elevated temperature, corrosion resistance and large melting points, shock resistance, erosion resistance and high creeping strength as well as thermal fatigue resistance. Inconel 718 is utilized in aerospace industry and aircraft engines. Titanium and nickel based alloys are commonly used in manufacturing aerospace engines. Equipments made for marine applications, petrochemical, nuclear reactors, food processing and other applications undergo many enhancements to increase their surface integrity as well as strength. At the elevated temperatures, they offer higher strength, wear and chemical resistance. Although because of poor thermal properties, suitable surface results are not received at the elevated temperatures due to friction and distortion caused by heat and variations in the microstructure.

The major problem found in heat resistant nickel alloys is their small thermal conductivity and large sensibility of adhesion which while machining enhances the influence of thermal and tool face factors. The alloy also contains carbides and abrasive particles that can cause high tool wear. Moreover they have an austenitic matrix that makes the work hard while machining. Additionally, the abrasive saw toothed boundaries are created due to localized shear in the chip production the handling of swarf tough. Moreover, the alloy can weld with the material of tool at the high temperature by machining. This affinity to produce built up edge while machining and abrasive carbides obstruct the machinability. It is due to elevated temperature above 1000oC and stress above 3450 MPA that increases the ring production, flank wear and notching.

Hastelloy C-2000 alloy is a nickel-chromium-molybdenum (Nickel-chromium-molybdenum) type alloy that is utilized in aerospace, marine and food treatment and chemical process plants. Problems and high expenses are anticipated in machining of alloy because it is made to retain its strength at the high temperatures. Therefore large efforts are made to discover a suitable method of machining of these alloys to improve its functionality. Machining of C-2000 alloy is done following the advanced methods in respect to the cutting force, tool life and surface hardness following the artificial neural network in the CNC milling machine.

A major complication occurs to manufacturers in the cutting throat competitive market due to the production conditions, low costs, great quality and target of high rates of construction. The accuracy of material dimension, tool wear, surface finish and service life of tool on the material eradication rate and cutting tool have been upgraded for improving the material performance in the influential conditions.

Hastelloy C-2000 is suitable because of its pattern to tolerate high temperatures resulting in high machining operations and cutting temperatures. The contact length of apparatus is small that creates large stress at the tool chip interface of the alloy product. The major attributes of the tool damage are flank wear, cracking, catastrophic, notching , chipping, plastic lowering and cutting boundary etc. The work hardening is another problem that causes large tool damage at the flank surface. The mechanical and thermal stress affect the residual stress of tool for a moment the thermal stress produces tensile stress for austenitic structure that is suppressed by the mechanical pressure. These pressures are more in the high temperature alloy machining that result in unwanted tensile stress. It is an essential production that the machining finishing process be of specific size, surface finish, tolerances, kind of surface productions and other actions. 

Hastelloy C-2000 introduces a new level of versatility in the Ni-Cr-Mo Alloys in resisting corrosion in the various conditions, blending excellent resistance to oxidizing conditions with great resistance to reducing conditions. Inconel C2000 shows an excellent performance for chemical process industrial applications.

Large content percentage of chromium is needed for resistance to oxidizing agents such as to ferric ions, cupric ions or dissolved oxygen. The reducing conditions on the other hand are dilute hydrochloric or sulfuric acids, need a significant content of molybdenum and tungsten.

Hastelloy C2000 solves the alloy design stress. High percentage of chromium is added with molybdenum and copper, adequate to offer excellent resistance to reducing conditions without compromise with metallurgical stability. It also offers greater resistance to pitting and crevice corrosion than alloy C276. Both alloys have similar forming, welding and machining properties.

C – 2000 alloy is produced in wire, sheet, plate, rod, bar, pipe, tubing, strip, flanges, foils and various other forms. The wrought forms of alloy are produced in solution heat processed condition generally.

Hastelloy C2000 alloy is produced under ASME Section VIII division 1 under code case 2240 and ASTM specification B – 564, B -574, B – 575, B – 619, B – 622, B – 626, B – 366. The DIN specifications are 17744 No. 2.4675 and NiCr23Mo16Cu. Alloy C – 2000 is specified as UNS N062000 however it has more limited composition for enhanced functionality. The enhancements are of importance that it has been commonly patented across the globe.

Welding of Hastelloy C-2000

Secured working environments should be maintained before welding. Welders should wear protected outfits, confined areas should be prevented and sufficient ventilation should be offered. The preferences of ANSI/ ASC Z 49.1 – 88, safety in welding and cutting are essential to follow. The weldable surfaces and nearby areas must be cleaned and degreased completely before welding. Nickel – Chromium – Molybdenum alloys have small penetration properties, hence sufficient joint access and nominal land are recommended. The interpass temperature is kept lower than 93oC or 200oF and wide heating should be prevented particularly on thin sections. The weld metal is viscous and hence some torch handling is often required. Increasing the current supply will not noticeably improve the fluidity of the weld puddle. Oxyacetylene and submerged arc welding are not preferred. Covered electrodes may need to be dried, although new electrodes should be kept in an oven at temperature limits of 121 oC to 204oC or 250oF to 400oF. The backing gas of 100% argon should be utilized for the root pass while gas tungsten arc or gas metal arc welding, for shielded metal arc welding, crushing of the back area of the root pass is important. For gas tungsten arc welding procedure, a stable current power supply, attained with a large frequency initialization and downslope control is preferred, torches equipped with gas diffuser lens offer the required gas coverage.

The post weld stress relieving at about 650oC or 1202oF is not sufficient for the nickel –chromium – molybdenum alloys, generally the post weld heat processing is not followed, however if they need to be stress relieved, a complete solution annealing at 1149oC or 2100oF is preferred and then water cooling is done.

For above 1% use of oxidizing shielding gas while gas metal arc welding, the grinding of the weld bead between every pass is preferred. The water quenched torches are preferred for gas metal arc spray transfer and synergic transfer. 

About Manufacturer

Heanjia Super-Metals produces Hastelloy C-2000 high temperature resistant alloy in the standard forms such as wire, sheet, strip, foil, pipe, flanges, tubing, plate, rod and other forms following the needs of challenging industries in the modern time. Other alloys of Hastelloy family manufactured are –  Hastelloy B2, Hastelloy B3, Hastelloy C4, Hastelloy C22, Hastelloy C276, Hastelloy C2000, Hastelloy G30 and Hastelloy X.