Processing of stainless steel burnt-wax castings 

Why Machining Precision Determines the Fate of a Stainless Steel Part

Stainless steel burnt wax casting processing is not just the final stage of production, but a critical process that separates a defective product from a high-quality product. In our practice, we have repeatedly encountered a situation where a perfectly cast billet made of steel grade 12Х18Н10Т (analogous to AISI 321) was sent for remelting solely due to errors at the stage of machining the sprues and surfaces. The lost-wax casting method makes it possible to obtain parts of the most complex geometry with minimal allowances, but it is precisely this feature that dictates strict requirements for cutting conditions and tool selection. Stainless steel has low thermal conductivity and a tendency to work hardening, which makes removing even 0.5 mm of metal an engineering task that requires a deep understanding of the physics of the process.

Many buyers make the mistake of evaluating a supplier only on the quality of castings, ignoring the fleet of metalworking equipment and the qualifications of technologists. The reality is that the foundry can be perfect, but if the machining department uses universal machines without adapting to tough alloys, you will end up with parts with a damaged surface layer structure. This leads to a decrease in corrosion resistance and fatigue strength under real operating conditions. We analyzed hundreds of batches and found a direct correlation between sprue removal method and complaint rate during the first year of product life.

In this guide, we will analyze in detail the technological nuances of post-processing, based on GOST standards and international ISO standards. You will learn why traditional methods often do not work with modern high-temperature alloys, what roughness parameters are actually achievable without electrochemical polishing, and how to avoid hidden defects. Our goal is to give you a tool to evaluate the competencies of a potential contractor, not just a theoretical overview. If you are planning to purchase critical components for the oil and gas or food industries, understanding these processes will save you budget on rework and equipment downtime.

Specifics of removing the gating system in investment casting

Gate removal is the first and most risky step in machining after heat treatment of castings. Unlike sand casting, where allowances amount to several millimeters, the lost-wax technology leaves only 0.3–0.8 mm of material for processing on stainless steel parts. This requires pinpoint precision when cutting feeders. The main problem is that the junction of the sprue and the part often has an altered metal structure due to the thermal solidification cycle. The wrong choice of cutting method leads to the formation of microcracks, which become centers of corrosion or fatigue failure under load.

In our production line, we have abandoned the use of simple abrasive cutting discs for critical parts made of austenitic steels. Although this method is cheap and fast, it creates a heat-affected zone (HAZ) where the metal is tempered or, alternatively, subjected to excessive work hardening. For steels of type 08Х18Н10 this is critical, since the passive oxide layer is damaged. We switched to a combination of methods: primary separation on CNC band saws followed by milling of the remainder with carbide cutters. This approach increases processing time by 40%, but reduces the crack rejection rate from 12% to less than 0.5%.

One of our clients, a manufacturer of pumping equipment for the chemical industry, experienced massive failure of pump housings after 6 months of operation. The investigation revealed that the subcontractor used the impact method to knock out the sprues to speed up the process. The shock wave created a network of microscopic defects in the casting body, which opened under the influence of the pulsating pressure of the working environment. This incident taught us that any impact contact is unacceptable for stainless steels. Now our technical specifications for all partners include a ban on impact methods for removing profits and sprues for parts operating under pressure above 1.6 MPa.

When choosing a cutting technology, it is necessary to take into account the wall thickness of the part. For thin-walled elements (less than 2 mm), we recommend using wire EDM, despite its high cost and low speed. This method completely eliminates mechanical pressure on the workpiece and does not create a HAZ. For massive components made of martensitic steels (for example, 20X13), mechanical processing on machining centers with high-pressure coolant supply directly to the cutting zone is permissible. It is important to remember: savings at the stage of removing sprues always result in multiple losses at the stage of warranty service.

Comparative Analysis of Gate Removal Methods

The choice of a particular method should not be based on the cost of the operation, but on the requirements of the final product for tightness and fatigue strength. If your part will operate in an aggressive environment or under cyclic loads, the only correct solution will be milling or electrical erosion. Asking the supplier for a report on sprue removal practices before entering into a contract is a simple question that will immediately weed out unscrupulous contractors.

Technology Challenges in Stainless Steel Machining

Machining stainless steel castings is fundamentally different from working with rolled products or forgings. The main reason lies in the heterogeneity of the structure of the cast metal. During the crystallization process, dendrites and segregation zones are formed, where the concentration of alloying elements may differ from the nominal one. This leads to the fact that the hardness of the metal varies within one part. The cutter, set to the optimal mode, suddenly encounters an area of ​​increased hardness, which causes chipping of the insert or vibration of the system. As a result, surface quality and geometric accuracy suffer.

Work hardening is the scourge of all technologists working with austenitic steels such as AISI 304 and 316. When the surface layer is deformed during the cutting process, the hardness of the metal can increase by 1.5–2 times. If the depth of the next pass is less than the thickness of the riveted layer, the tool begins not to cut, but to slide along the surface, generating a huge amount of heat. This accelerates tool wear and degrades surface quality. In our practice, we strictly control the depth of material removal: each pass must be greater than the previous work hardening. Ignoring this rule resulted in one batch of shafts being rejected due to a 0.05 mm discrepancy in diameter after finishing.

The low thermal conductivity of stainless steel exacerbates the problem. The heat generated in the cutting zone is not transferred to the chips or the body of the part, but is concentrated on the cutting edge of the tool. Temperatures can reach 900–1000°C, which exceeds the heat resistance limit of many hard alloys. The solution to this problem lies in the organization of cooling. Conventional pouring of coolant from above is often ineffective, since chips and centrifugal forces prevent the liquid from penetrating into the contact zone. We have implemented a 70 bar coolant system through channels in the tool. This made it possible to increase the durability of the cutters by 3 times and improve the surface cleanliness.

Vibrations are another hidden enemy of quality. Cast workpieces often have complex shapes and uneven mass distribution, making them prone to resonance during processing. The rigidity of the fastening becomes a critical factor. Using a standard vice is often not enough; We use individual technological equipment with vacuum clamping or hydraulic chucks for thin-walled parts. We once lost a week of production time trying to machine a batch of valve bodies until we realized that the vibration was caused by insufficient rigidity of the adapter bushing in the machine spindle. Replacing the bushing with a precision one solved the problem instantly.

Critical parameters of cutting conditions

• Cutting speed (Vc):For stainless steel it should be lower than for carbon steel, usually in the range of 60–120 m/min depending on the grade of alloy. Exceeding the speed leads to rapid burnout of the tool.
• Feed (fz):It should be sufficient so that the tool “bites” into the material rather than rubbing against it. The minimum chip thickness must exceed the radius of the cutting edge.
• Depth of cut (ap):It is recommended to use medium depths, avoiding values that are too small, which causes hardening, and too deep, which causes vibration.
• Tool geometry:Milling cutters with a positive rake angle and a sharp cutting edge are preferred to reduce cutting forces. TiAlN coatings are required for operation at high temperatures.

Understanding these parameters allows you to predict tool life and batch quality consistency. When ordering machining services, be sure to ask whether the contractor uses specialized stainless steel tooling or general-purpose solutions. The difference in cost may be minimal, but the difference in the quality of the finished product is colossal.

Achieving the required roughness and finishing

Requirements for the surface roughness of stainless steel parts are often dictated not only by aesthetics, but also by functional needs. In the food and pharmaceutical industries, hygienic design standards (such as EHEDG or 3-A Sanitary Standards) require that the surface be free of porosity and irregularities where bacteria can accumulate. The Ra parameter (arithmetic mean deviation of the profile) for such parts should not exceed 0.8 µm, and sometimes 0.4 µm. Achieving such values by finishing milling or turning methods is extremely difficult and economically impractical for most series.

The main method of achieving high surface cleanliness remains grinding and polishing. However, there are many pitfalls here. Mechanical polishing with abrasive belts or wheels can result in uneven material removal, especially on complex curved surfaces common in investment casting. Corners and inner radii are often left untreated, creating stagnation areas. In addition, intense friction heats the surface, which can cause discoloration (tarnish) and a decrease in corrosion resistance due to burnout of chromium in the surface layer.

We are actively introducing methods of electrochemical polishing (electropolishing) as an alternative to mechanical polishing. This process is based on the anodic dissolution of a metal in an electrolyte under the influence of an electric current. The projections of microroughness dissolve faster than the depressions, which leads to a smooth surface without mechanical contact. The advantages are obvious: no hardening, removal of contaminated surface layers, increased corrosion resistance and ideal treatment of hard-to-reach areas. For complex shaped parts produced by casting, electropolishing is often the only way to guarantee the required quality over the entire surface.

However, electropolishing has its limitations. It does not correct geometric deviations of the shape (flatness, coaxiality), but only smoothes out the microrelief. Therefore, it always follows high-quality mechanical processing. It is also important to properly prepare the surface: the presence of scale, oils or deep scratches from previous operations can lead to an uneven process and stains. In one of the projects, we were faced with the fact that a batch of fittings became covered with dull spots after electropolishing. The analysis showed that the lubricant used in milling was poorly removed before polishing. The introduction of ultrasonic washing before the galvanic line completely eliminated the defect.

For products where not only smoothness is important, but also a decorative appearance (architectural elements, fittings), a combined approach is used: preliminary grinding with P120-P240 grain, then mechanical polishing with felt wheels with paste and final electropolishing. This cascade of operations allows you to remove casting marks and obtain a mirror shine. It is important to control the thickness of the layer being removed: the total removal of metal at all stages should not exceed the permissible deviations in the dimensions of the part indicated in the drawing.

Quality control and compliance with international standards

Quality assurance in the production of stainless steel parts is impossible without a multi-level control system. Simply checking the dimensions with a caliper is not enough. Cast parts are susceptible to internal defects, which can only appear under load or in an aggressive environment. Our inspection system includes incoming chemical composition inspection (spectral analysis), non-destructive testing (NDT) and geometry acceptance testing. Each stage is documented and accompanied by certificates that are handed over to the customer.

Non-destructive testing is mandatory for critical applications. We use three main methods: visual (VT), capillary (PT) and radiographic (RT). Visual inspection allows you to identify obvious surface defects: cracks, pores, underfilling. Penetrant testing (color flaw detection) reveals microcracks invisible to the eye, which is critical for hermetic connections. Radiographic testing (X-ray) makes it possible to look inside the metal and detect gas pores, slag inclusions or shrinkage cavities. According to ASTM E125 or ISO 10675, we classify defects and decide whether to reject or repair.

Particular attention is paid to monitoring geometric parameters. Investment casting provides high precision, but metal shrinkage is a stochastic process. The use of coordinate measuring machines (CMMs) allows you to build a 3D model of a real part and compare it with a CAD model of a drawing. This reveals deviations in the shape and location of surfaces that cannot be seen with hand tools. In our practice, there was a case when a batch of turbine impellers passed visual inspection, but the CMM revealed a deviation of the blade profile by 0.1 mm, which would lead to imbalance and destruction of the assembly at high speeds. Timely detection saved the client from a serious accident.

Product certification confirms compliance with international standards. To operate on the European market, CE marking and compliance with the PED (pressure equipment) or MD (machinery) directives are required. For the Russian and EAEU markets, GOST certificates of conformity and the CU TR declaration are required. The presence of an ISO 9001 certificate from a manufacturer indicates well-established quality management processes, but does not replace specific product testing. We recommend that customers always request Mill Test Reports for each metal melt and a final batch inspection report. The absence of these documents is a red flag indicating possible problems with traceability of materials.

Practical experience: from theory to real projects in the energy and petrochemical industries

Theoretical knowledge of metallurgy and processing is important, but it is practical implementation in real production conditions that distinguishes a reliable partner from an ordinary supplier. A striking example of this approach is the company’s activitiesWuxi Kaisheng Electric Power and Petrochemical Equipment Co.,Ltd" Специализируясь на разработке и производстве сложного теплообменного оборудования, компания успешно интегрирует описанные выше технологии в выпуск высокотехнологичной продукции.

В портфеле решений ООО «Уси Кайшэн» — титановые кожухотрубные теплообменники, высоконапорные агрегаты стандарта ASME, а также гофрированные трубные пучки из нержавеющей стали 316 и сплавов N06625. Производство таких изделий требует безупречного владения методами обработки трудных материалов. Например, при изготовлении трубных решеток из стали 321 или медно-никелевых сплавов C70600 критически важно соблюдать режимы резания, чтобы избежать наклепа и сохранить коррозионную стойкость, особенно учитывая, что оборудование предназначено для работы в агрессивных средах опреснения морской воды или нефтепереработки.

Опыт компании подтверждает, что заявленные в статье принципы — от выбора метода удаления литников до финишной электрополировки — напрямую влияют на эксплуатационные характеристики готовых узлов. Воздушные охладители и котлы-утилизаторы, выпускаемые предприятием, сертифицированы по строгим международным стандартам PED и ASME, что было бы невозможно без глубокого понимания физики процессов обработки нержавеющих, легированных сталей и цветных металлов. Клиенты компании, работающие в судостроении, химической промышленности и энергетике, получают не просто детали, а готовые инженерные решения, где каждый микрометр шероховатости и каждая структура металла проверены на соответствие условиям экстремальных давлений и температур.

Frequently Asked Questions

Какова минимальная толщина стенки, возможная при литье и последующей обработке?

Технологически возможно отлить стенки толщиной от 0,5 мм, однако для последующей механической обработки мы рекоме ндуем проектировать стенки не тоньше 1,5–2,0 мм. Более тонкие стенки крайне сложно закрепить без деформации, а риск сквозного пробоя или нарушения геометрии при снятии припуска возрастает экспоненциально. Если конструкция требует тонких стенок, рассмотрите возможность использования литья с минимальной пост-обработкой или переход на листовую сварную конструкцию.

Можно ли достичь шероховатости Ra 0.4 только механическим путем?

Теоретически да, используя алмазное точение или суперфиниш, но для деталей сложной формы, полученных литьем, это экономически нецелесообразно и технологически трудно реализуемо. Стандартный промышленный метод получения Ra 0.4 для нержавеющих сталей — это комбинация чистового шлифования и электрополировки. Попытка получить такое качество только фрезерованием приведет к удорожанию детали в 5–10 раз из-за огромного времени обработки и расхода инструмента.

Как влияет термическая обработка на размеры детали после литья?

Термическая обработка (закалка, отпуск,.solution annealing) неизбежно вызывает изменение линейных размеров детали из-за снятия внутренних напряжений и фазовых превращений. Для аустенитных сталей изменение может составлять 0,1–0,3% от линейного размера. Поэтому все критические размеры должны обрабатыватьсяafterтермообработки. Закладывать припуск на термообработку без учета этого фактора — грубая ошибка, ведущая к браку. В нашем техпроцессе мы всегда оставляем увеличенный припуск на черновую обработку до термообработки и минимальный — на чистовую после неё.

Какие марки нержавеющей стали наиболее сложны в обработке?

Наиболее сложными считаются аустенитные стали с высоким содержанием никеля и молибдена (например, AISI 316Ti, 904L) и дуплексные стали (2205). Они обладают высокой вязкостью, склонностью к наклепу и низкой теплопроводностью. Мартенситные стали (типа 420) после закалки становятся очень твердыми и требуют использования твердосплавного инструмента или шлифования. Ферритные стали (430) обрабатываются легче, но склонны к образованию нароста на резце. Выбор режимов резания должен строго соответствовать конкретной марке сплава.

Conclusion and recommendations for choosing a supplier

Обработка литья по выжигаемым восковым моделям из нержавейки — это симбиоз искусства литейщика и мастерства оператора станков с ЧПУ. Качество конечного продукта зависит от каждого звена этой цепи: от правильности изготовления восковой модели до финишной полировки. Ошибки на любом этапе необратимы и ведут к потере дорогостоящего материала и времени. Рынок переполнен предложениями, но лишь единицы производителей обладают полным циклом контроля и глубоким пониманием металлургии нержавеющих сталей.

При выборе партнера обращайте внимание не на красивую презентацию, а на технические детали: какое оборудование используется для удаления литников, есть ли в парке станки с подачей СОЖ под высоким давлением, проводится ли рентген-контроль каждой партии. Спросите о случаях брака в прошлом и о том, как они были устранены. Честный поставщик расскажет о своих ошибках и выводах, так как именно опыт преодоления проблем формирует настоящую экспертизу. Избегайте тех, кто обещает «любую сложность за копейки» — в металлургии чудес не бывает, есть только технологии и труд.

Мы готовы применить наш 15-летний опыт для реализации ваших проектов любой сложности. От прототипирования до серийного выпуска тысяч единиц продукции, мы гарантируем соблюдение допусков, требуемую шероховатость и полную документальную поддержку. Свяжитесь с нами сегодня, чтобы обсудить ваш чертеж и получить технико-коммерческое предложение с расчетом сроков и стоимости.Contact us todayдля консультации с ведущим инженером-технологом.