金属和材料加工常见问题解答

Carbon steel components have been routinely annealed or heat treated in nitrogen-hydrogen atmospheres to relieve stress, alter microstructure and/or improve surface appearance for a number of years. The flow rate and composition of nitrogen-hydrogen atmosphere to be used for annealing components in furnaces are usually determined by a trial and error approach. Once the atmosphere flow rate and composition that produces parts with acceptable quality have been determined, they are generally fixed for future annealing operations. Although the composition of nitrogen-hydrogen atmosphere introduced into a furnace does not change with time, the true reducing or oxidizing potential of the atmosphere inside the furnace changes continuously with time due to leaks and drafts in the furnace, desorption of impurities such as moisture from the surface of components or decomposition of lubricant present on the surface of components being annealed. This continuous change in reducing or oxidizing potential of the atmosphere inside the furnace provides a great difficulty to commercial heat treaters and parts producers to produce annealed components with good and consistent quality and compete effectively in the global market. Therefore, to provide operational flexibility to commercial heat treaters and parts producers in terms of (1) controlling the true reducing or oxidizing potential of the atmosphere inside the furnace and (2) improving quality of annealed components, Air Products has developed an advanced control and monitoring system.

In the metals processing industry, heat treatment applications are required for producing parts with the desired mechanical and surface properties, as well as for stress relief after mechanical deformation. Today some companies use exothermic or endothermic generators or ammonia dissociates to create the necessary atmospheres. Compared with atmospheres composed of technical gases such as nitrogen and hydrogen, these generated gases have serious disadvantages.

All grades of stainless steels are iron-based alloys with significant percentages of chromium. Typically, stainless steels contain less than 30% chromium and more than 50% iron. Their stainless characteristics stem from the formation of an invisible, adherent, protective and self-healing chromium-rich oxide (Cr₂O₃) surface film. While stainless steels are resistant to rusting at room temperatures, they're prone to discoloration by oxidation at elevated temperatures due to the presence of chromium and other alloying elements such as titanium and molybdenum.

Factors that contribute to increased oxidation include high dew points, high oxygen and oxides of lead, boron, and nitrides on the surface. For bright stainless steels, process them in a highly reducing atmosphere with a dew point lower than –40°F and a minimum of 25% hydrogen.

The green color that you see on stainless steel parts is chromium oxide (Cr₂O₃). It forms when there is too much oxygen and/or moisture in the furnace atmosphere, which is usually caused by a water leak, poor atmosphere tightness, or overly low flow rates of atmosphere gas. A dark green-brown color indicates significant levels of free oxygen inside the furnace originated by a large air leakage.

In addition to the traditional steel and copper test, some companies run a piece of stainless through the furnace to check for high moisture and oxygen levels. A better and more precise way of measuring moisture and oxygen levels is to install an oxygen analyzer and dew point meter. It's inexpensive and highly accurate. If a green oxide film is forming on your stainless steel parts, that's an indication that the furnace or atmosphere is not optimized.

Dezincification is typically defined as the leaching of zinc from copper alloys in an aqueous solution. In thermal processing of brasses (and other zinc-containing alloys), dezincification is the removal of zinc from the metal substrate during thermal processes, like brazing and annealing, typically due to the very low vapor pressure of zinc in the alloys. Dezincification can result in excessive furnace dusting, zinc vapors alloying with other metals, and in extreme cases, loss of alloy properties.

While eliminating dezincification is not always possible, it can be reduced during thermal processing. Controlling temperature, time at temperature, and the furnace atmosphere's reducing potential can help minimize dezincification and improve your thermal processing. However, understanding which variables to change can be a challenge. Air Products' industry specialists, experienced in thermal processing, can help pinpoint the variable(s) that you can regulate to help lower costs and improve productivity by minimizing dezincification.

Bright annealing of steels requires conditions that are reducing to steel oxides. Traditionally, the Ellingham diagram has been used to predict the conditions that correspond to oxidation of pure metals or reduction of their oxides. This method can be used to predict the conditions that should be reducing to iron oxides and the oxides of the alloying elements added to steels, such as chromium oxide when stainless steels are considered. This traditional approach is not precise because it only uses thermodynamic data for pure metals and their oxides—it ignores the fact that iron and alloying elements form a solid solution. In addition, you can only determine the approximate equilibrium partial pressure ratio of hydrogen and water vapor for oxidation of a specific metal at a particular temperature.

Alternatively, you can use more accurate and convenient diagrams for steels and other alloys, which are created with the help of modern databases and computer programs, such as FactSage™ (thermochemical software and database package developed jointly between Thermfact/CRCT and GTT-Technologies) or Thermo-Calc software. Using the oxidation-reduction curves, presented as dew point of pure hydrogen or nitrogen-hydrogen atmospheres versus temperature, you can quickly select the atmosphere for annealing steels without formation of oxides. The diagram in Figure 1 was calculated using FactSage. This diagram shows that oxidation-reduction curves for Fe-18%Cr and Fe-18%Cr-8%Ni systems representing stainless steels are higher than the corresponding Cr/Cr₂O₃ curves. For alloys (e.g. steels), you can achieve more precise calculations using thermodynamic data from both the pure substances (i.e. pure metals and oxides) and solutions databases. Such diagrams can be produced specifically for the steels of interest and variety of atmosphere compositions.

These methods can help you troubleshoot and optimize your annealing operation by balancing hydrogen usage versus product quality.

Figure 1:

流量计的尺寸必须适合每个特定应用、气体类型、气体压力和工作范围。 首先，确保您的流量计已针对您要计量的气体的比重进行了校准。 检查流量计的标签或玻璃管或致电制造商确定。 其次，仅在校准的压力下操作流量计。 例如，校准为 80 psi 且读数为 1000 scfh 的可变面积流量计如果在 40 psi 下运行，则实际上只能提供 760 scfh。 这是 24% 的错误！ 第三，为了获得最佳精度并留出调整空间，请调整流量计的大小，使您的正常流量在满量程的 30%–70% 范围内。 这三个步骤将有助于确保您能够很好地控制气体流量，并最终控制您的过程。

传统上，高压气瓶一直是中低容量用户的供气方式。 这使公司容易受到与移动气缸和暴露于高压相关的安全风险的影响。 整合到一个集中的微型散装系统消除了处理钢瓶的需要，并降低了产品混淆的风险。 进一步的好处包括减少接触高压容器和减少交通拥堵，减少供应商的交货频率。 空气产品公司开发了微型批量供应选项，作为用于氮气、氩气、氧气和二氧化碳供应的高压气瓶的经济、可靠的替代方案。 除了高效灵活的存储系统外，还提供创新的管道解决方案来帮助您从气瓶平稳过渡到微型散装。

Since the reducing potential of a hydrogen-based furnace atmosphere is defined by the ratio of pH₂O, the first answer that comes to most people’s minds is “yes.” And, in some cases, they are correct. Lower dew point readings (lower pH₂O) lead to more reducing conditions and, in many cases, better furnace atmosphere performance. However, there are situations where that is not always the case. One example of that is hydrogen-based belt furnace atmospheres where the dew point can reach values drier than –50°F or even –60°F under certain conditions. The reducing potential of this atmosphere is more than enough for the typical parts processed, but it can lead to unnecessarily strong reducing conditions that decrease belt life. Another example might be a brazing atmosphere that is too reducing and prone to excessive braze flow. Air Products’ new Atmosphere Humidification System allows for precise and consistent moisture additions to furnace atmospheres for just the right amount of moisture to improve belt life performance and/or braze flow while still maintaining adequate reducing conditions for the sintering or brazing operations being performed.

Dezincification is typically defined as the leaching of zinc from copper alloys in an aqueous solution. In thermal processing of brasses (and other zinc-containing alloys), dezincification is the removal of zinc from the metal substrate during thermal processes, like brazing and annealing, typically due to the very low vapor pressure of zinc in the alloys. Dezincification can result in excessive furnace dusting, zinc vapors alloying with other metals, and in extreme cases, loss of alloy properties.

While eliminating dezincification is not always possible, it can be reduced during thermal processing. Controlling temperature, time at temperature, and the furnace atmosphere's reducing potential can help minimize dezincification and improve your thermal processing. However, understanding which variables to change can be a challenge. Air Products' industry specialists, experienced in thermal processing, can help pinpoint the variable(s) that you can regulate to help lower costs and improve productivity by minimizing dezincification.

All grades of stainless steels are iron-based alloys with significant percentages of chromium. Typically, stainless steels contain less than 30% chromium and more than 50% iron. Their stainless characteristics stem from the formation of an invisible, adherent, protective, and self-healing chromium-rich oxide (Cr₂O₃) surface film. While stainless steels are resistant to rusting at room temperatures, they're prone to discoloration by oxidation at elevated temperatures due to the presence of chromium and other alloying elements such as titanium and molybdenum.

Factors that contribute to increased oxidation include high dew points, high oxygen and oxides of lead, boron, and nitrides on the surface. For bright stainless steels, process them in a highly reducing atmosphere with a dew point lower than –40°F and a minimum of 25% hydrogen.

For sintering and brazing atmospheres in a continuous belt type furnace with open ends, you must follow NFPA 86 Standard for Ovens and Furnaces. Typically, atmospheres containing greater than 4% hydrogen in nitrogen are considered flammable. In fact, any mixed atmosphere—even if it contains less than 4% hydrogen—is considered “indeterminate” and must be treated as if it were flammable.

NFPA 86 recommends you satisfy the following conditions before introducing any flammable or indeterminate atmosphere into the furnace:

• At least one zone of the furnace must be hotter than 1400°F.
• The furnace must be purged with an inert gas until the atmosphere analysis indicates it’s below 50% of its LEL (lower explosive limit). General recommendation is to use five volume changes of inert gas flow.
• There must be visible indication of purge flow. Plus, purge piping should have normally open solenoid valves.
• The atmosphere system should be designed with interlocks so the flammable gases are shut off using normally closed solenoid valves in the event of power failure, a temperature drop below 1400°F, or insufficient flow of the main atmosphere component.

这取决于你的过程。 多年来，用于金属加工的氮基气氛已被成功证明，并且由于熔炉对各种材料和表面需求的广泛要求，气体混合物的使用现在已成为行业标准。 由于混合物中存在额外的还原性或反应性成分，不同的产品可以耐受炉内气氛中不同浓度的氧化性成分。 出于这个原因，可以容忍使用现场产生的带有残余氧气的氮气。 通过了解您的氧气耐受水平，我们可以帮助您降低成本。

A simple copper/ steel test can differentiate oxidation by air (O₂) or water (H₂O). The test is performed by sending a piece of clean bright copper strip alongside a piece of clean carbon steel strip through the continuous furnace and observing the oxidation on each test coupon. Take care to keep the furnace temperature below 1981˚F, the melting point of copper. The steel strip will discolor or oxidize if the atmosphere has an air or water leak; however, the copper strip will only oxidize if an air leak is present. You can use this test for nitrogen-based or generated type atmospheres like endothermic or dissociated ammonia. And it can be done without oxygen or dew point analyzers.

Yes, leaks in any pressurized high-purity gas line can cause intermittent oxidation. There are several possible causes. One is through retrodiffusion—the movement of impurities from the surrounding air to a high-pressure, low-impurity gas houseline. This is driven by concentration gradients, not pressure gradients, and is aggravated by changes in flow rate, pressure or piping temperature.

Air Products industry specialists can help you determine the cause of your problem. Since the oxidation is intermittent, you’ll need to continuously monitor your nitrogen houseline for leaks with a trace oxygen analyzer. For combustible gas lines, a combustible gas sniffer can also be used. Once impurities are found, the source of the leak can be identified using various techniques, including soap bubble testing, static pressure testing or helium mass spectrometry. Leaks often occur in weld cracks, mechanical joints, valve packing and loose fittings.

Carburizing and other carbon control atmospheres all require a source of CO to facilitate the diffusion of carbon into the surface of the metal. One source is through endothermic atmosphere generation, in which air and natural gas are reacted in an external generator to form a gas composed of 20% CO, 40% H₂, and 40% N₂, with trace amounts of CO₂ and moisture.

Another source of CO is the introduction of a blend of 40% nitrogen and 60% methanol into the furnace, which forms a gas of the same composition produced endothermically. The heat of the furnace dissociates the methanol (CH₃OH) into CO and H₂, which then blends with the nitrogen. Here's how to calculate the amount of methanol needed. For 1000 scf of atmosphere, as an example, 40% or 400 scf will be nitrogen, according to the ratios above. The remaining 60% or 600 scf will be made up of dissociated methanol. Since one gallon of methanol dissociates into approximately 240 scf of gas, 2.5 gallons of methanol would be needed to dissociate into the required 600 scf of atmosphere.

Refractories are affected by atmospheres in several ways. Although stable at room temperature, a number of oxides are reduced in the presence of hydrogen or free carbon at elevated temperatures—thus shortening their lives. The customer's process and desired output dictate the design atmosphere. However, crystallography of the ceramic material will have a major impact on its resistance to that atmosphere. By understanding the effects of atmosphere gases on refractories and by selecting refractories that are more stable at operating temperatures and in the presence of specific gas species, you can enhance the performance of your furnace. Air Products' engineers can work with you to optimize your process.

This is a question that comes up frequently. When troubleshooting for oxidation in a continuous furnace atmosphere, it's important to measure both oxygen level and dew point. Here's why.

The dew point is a measure of the moisture content of a gas and is the temperature at which water vapor in a sample gas starts to condense. Oxygen concentration is simply that—a measure of the partial pressure of oxygen.

When a gas sample is extracted from the hot zone of a furnace for analysis, reactive gases like H₂, CO, or CₓHᵧ have already combined with any O₂ present to produce moisture and other gaseous components. As a result, depending on the furnace temperature and how the sample is obtained, your analyzer will often display a low oxygen level. In most applications, a low oxygen level and a low dew point are required to control the process and prevent oxidation.

When checking a continuous furnace, oxidation in the preheat section has a matte or frosted appearance and is usually caused by air infiltration from the entrance of the furnace. Hot zone oxidation may cause scaly or blistered parts. This generally occurs from elevated moisture or oxygen levels due to improper atmosphere balance or water/air leaks in the cooling zone. Cooling zone oxidation typically results in a smooth, sometimes shiny discoloration—poor curtain design, excessive belt speed, water leaks, or insufficient atmosphere flow rates are possible causes.

In batch furnaces, start by identifying the oxidant causing the problem. Flowing nitrogen and measuring the oxygen and moisture levels can give an indication of the oxidant involved. Then a review of typical leak sources, such as seals, fittings, unions, and weld joints, usually leads to discovery of the leak source.